Varying Doses of Evening Caffeine Ingestion Have Different Effects on Rowing Ergometer Performance, Sleep Quality and Wakefulness Scores
Phase: NA
Conditions: Healthy Volunteers, Caffeine, Sleep Quality
Sponsor: Ulaç Can YILDIRIM

 
 
 
 
O#$c$al T$tle: Vary%ng Doses of Even%ng Ca1e%ne Ingest%on Have 
D%1erent E1ects on Row%ng Ergometer Performance, Sleep Qual%ty 
and Wakefulness Scores 
 
 
 
NCT Number: Not yet ass)gned 
 
 
 
Date of Document: 24 July 2025 

 
 
Varying Doses of Evening Caffeine Ingestion Have Different Effects on 
Rowing Ergometer Performance, Sleep Quality and Wakefulness 
Scores 
1 
Abstract 
2 
This study investigated the dose-dependent effects of evening caffeine ingestion on rowing 
3 
performance, sleep quality, and daytime sleepiness in trained male rowers. Employing a double-blind, 
4 
randomized, crossover design, 13 university-level rowers completed four 2000-meter time trials under 
5 
placebo (PLA), low (3 mg/kg, LDC), moderate (6 mg/kg, MDC), and high (9 mg/kg, HDC) caffeine 
6 
conditions. Performance metrics, heart rate, and subjective sleep assessments were collected. Results 
7 
indicated significantly improved rowing times and power output with HDC and MDC compared to 
8 
PLA (p < 0.05), with HDC yielding the most notable enhancements (d = 0.4030.41). However, these 
9 
ergogenic benefits were accompanied by significantly impaired sleep quality and elevated daytime 
10 
sleepiness in both HDC and MDC groups (p < 0.01, d = 1.331.5). Notably, adverse effects such as 
11 
headache, insomnia, and gastrointestinal discomfort were predominantly reported in the HDC 
12 
condition (p < 0.05). Although LDC offered mild performance improvements with minimal sleep 
13 
disruption, only the high dose condition exhibited large physiological and perceptual trade-offs. These 
14 
findings indicate a clear dose3response relationship, wherein higher evening caffeine intake improves 
15 
performance but have detrimental effects on sleep and recovery markers. Coaches and athletes should 
16 
carefully balance caffeine dosing against potential recovery costs, especially in evening training or 
17 
competition contexts. 
18 
1 
Introduction 
19 
The use of caffeine, a component of the methylxanthine group, as an ergogenic aid is widely preferred 
20 
by athletes to increase physical and cognitive performance (Pickering & Grgic, 2020). More than 75% 
21 
of athletes in various branches take caffeine supplements to gain an advantage before or during 
22 
competition (Del Coso et al., 2011; Van Thuyne et al., 2005). Caffeine use has been reported to 
23 
increase, especially in individual and aerobic-based branches. Additionally, among the analyzed 
24 
athletes, rowers appear to be among the highest caffeine users (Aguilar-Navarro et al., 2019). 
25 
The desired form of caffeine can be chosen, including coffee, capsule, gum, bar, gel and aerosol 
26 
(Wickham & Spriet, 2018). Acute caffeine intake has been confirmed to have a positive effect on 
27 
athletic performance related to parameters such as cardiovascular endurance (Southward et al., 2018), 
28 
anaerobic endurance (Grgic, 2018), movement speed (Raya-Gonzalez et al., 2020), power (Grgic et 
29 
al., 2018) and muscular endurance during resistance exercises (Polito et al., 2016). The most widely 
30 
known mechanism of caffeine's ergogenic effects is through antagonism of adenosine receptors, which 
31 
in turn reduces the concentration of several central nervous system neurotransmitters, including 
32 
serotonin, dopamine, acetylcholine, norepinephrine, and glutamate (Fredholm, 1995). Caffeine 
33 
compounds inhibit the known effect of adenosine by binding to adenosine receptors due to their similar 
34 
structure to adenosine. It is known to improve aerobic-muscle endurance performance by increasing 
35 
the release of neurotransmitters that promote wakefulness (Astorino & Roberson, 2010; Guest et al., 
36 
2018; Higgins et al., 2016; Mohr et al., 2011). 
37 

 
2 
Rowing is a sport that is likely to be affected by natural circadian (24-hour) or diurnal (time of day) 
38 
changes because significant muscle strength is required especially in the drive phase of the rowing 
39 
stroke(Steinacker, 1993). It has been thought by researchers that the ergogenic effects of caffeine can 
40 
be used to compensate for possible decreases in performance. However, despite its potential to improve 
41 
performance, caffeine can cause side effects such as headache, nausea, insomnia or anxiety (Wikoff et 
42 
al., 2017). In a meta-analysis examining the side effects that occur in athletes after caffeine 
43 
supplementation, it was reported that there was a 34% higher probability of side effects after consuming 
44 
low and moderate doses of caffeine (de Souza et al., 2022). The same meta-analysis study emphasized 
45 
that heart palpitations and sleep problems were the most frequently reported side effects. Considering 
46 
the negative effects of sleep deprivation on exercise performance (Bonnar et al., 2018; Fullagar et al., 
47 
2015), it is important to identify the conditions that pose a risk to athletes' sleep quality. The effects of 
48 
afternoon caffeine intake on sleep efficiency the following night have been the focus of some 
49 
researchers. Moderate doses (6 mg/kg) of caffeine taken before cycling exercises at approximately 5:00 
50 
PM delayed falling asleep and shortened total sleep time (Miller et al., 2014). Another similar study 
51 
reported that 6 mg/kg of caffeine taken before an 800-meter running test at 8:00 PM reduced sleep 
52 
efficiency (Ramos-Campo et al., 2019). In the same study negative feedback was received from athletes 
53 
for subjective sleep parameters such as <sleep quality,= <calm sleep,= <ease of falling asleep,= and 
54 
<feeling refreshed after waking=. Other studies involving rugby players have also found negative or 
55 
neutral findings on caffeine-related sleep quality (Caia et al., 2021; Dunican et al., 2018) In the study 
56 
conducted by Caia et al. (2021) post-competition salivary caffeine concentrations of athletes who 
57 
continued their usual caffeine consumption before or during the competition were analyzed. A 
58 
moderate negative relationship was found between the increase in caffeine levels and sleep onset delay 
59 
and sleep efficiency. This effect of caffeine on sleep efficiency is attributed to the time it takes to be 
60 
metabolized. After caffeine is taken, its concentration in the blood plasma typically reaches its highest 
61 
level within 60 minutes and it takes approximately 4-6 hours for half of the initial dose to be 
62 
metabolized (Burke, 2008). Therefore, caffeine consumed before going to bed reduces sleep duration 
63 
and sleep efficiency (Drake et al., 2013). In addition, it can be said that caffeine reduces sleep efficiency 
64 
due to the tendency of the individual to temporarily increase the number of awakenings during the 
65 
sleep period (McHill et al., 2014). Therefore, total sleep time will also decrease. 
66 
There are conflicting studies on the optimum caffeine dose in caffeine consumption (Clarke et al., 
67 
2016; Spriet, 2014). In addition, randomized studies in the literature where different doses are observed 
68 
in the same sample group are limited. Moreover, verification of the effect of different caffeine doses 
69 
on sleep quality will provide valuable information especially for athletes who use caffeine before 
70 
evening training or competitions (Burke, 2017). Moderate to high doses (639 mg/kg) of caffeine 
71 
significantly improve short-duration, high-intensity rowing performance by reducing the time required 
72 
to complete the distance on a 2000 m rowing ergometer (Bruce et al., 2000). Similarly, 6 mg/kg of 
73 
caffeine is observed to significantly improve mean power output in 2000 m rowing performance 
74 
(Gharaat et al., 2020). Contrary to these findings, in the study conducted by Skinner et al. (2010), it 
75 
was concluded that 2, 4 and 6 mg/kg of caffeine did not provide any improvement in performance. The 
76 
conflicting results regarding caffeine and rowing performance suggest that further randomized studies 
77 
are needed. Furthermore, the fact that low-dose caffeine supplementation (f3 mg/kg) generally 
78 
improves athletic performance, as well as the low risk of side effects reported (Spriet, 2014), suggests 
79 
that the negative effects of caffeine on sleep quality can be minimized. This is supported by a study 
80 
conducted by Filip-Stachnik et al. (2021) on judokas. After consuming 3 mg/kg of caffeine in the 
81 
evening (19:00), they concluded that low-dose caffeine did not cause a significant deterioration in 
82 
objective sleep parameters after the following night's sleep analysis of athletes who trained. However, 
83 
the lack of studies in the literature that evaluate the effects of caffeine use on performance, as well as 
84 
the effects on sleep quality the night following use and the level of alertness the next day, is striking. 
85 

 
3 
In this context, the aim of the present study was to examine the effects of different doses of caffeine 
86 
taken in the evening on (1) ergometric rowing performance, (2) sleep quality, and (3) alertness levels. 
87 
This research raised the following hypotheses: I - different doses of caffeine will produce different 
88 
performance and sleep-related responses. II - low, medium and high doses of caffeine supplements 
89 
taken in the evening will improve athletes' performance on the rowing ergometer (time trial and average 
90 
power output). III - caffeine intake, except for low doses, will negatively affect sleep quality and 
91 
increase sleepiness levels. 
92 
2 
Material Methods 
93 
2.1 
Study Design 
94 
The research employed a double-blind, crossover design with randomization. Participants engaged in 
95 
2000-meter indoor rowing time trials during four separate laboratory visits. Time trials were conducted 
96 
under four primary testing conditions involving the intake of: 3 mg/kg caffeine, 6 mg/kg caffeine, 9 
97 
mg/kg caffeine or placebo. All testing sessions were carried out in a manner that was both randomized 
98 
and counterbalanced. Participants were randomized by the independent researcher to the intervention 
99 
or placebo groups using online research randomizer (https://www.randomizer.org/) which uses the 
100 
<Math.random= method within the JavaScript programming language to generate its random numbers 
101 
(Urbaniak and Plous, 2013). 
102 
Every time trial was performed using a Concept II Model D Rowing ergometer (Concept II, 
103 
Morrisville, VT, USA), with both the completion time and average power output (measured in watts) 
104 
being documented. Additionally, heart rate measurements were taken during the tests utilizing a Polar 
105 
Team Pro system equipped with an H10 sensor (Polar Electro OY, Kempele, Finland). The testing 
106 
sessions were spaced at least 72 hours apart, with a maximum interval of seven days, to guarantee 
107 
adequate washout of treatments and sufficient recovery time.  
108 
Each participant's warm-up routine was documented during the initial trial and then consistently 
109 
reproduced in the subsequent trials. Testing sessions were uniformly scheduled in the evening, between 
110 
19:00 and 20:00, and at identical times for each participant to mitigate the influence of circadian 
111 
rhythms. Upon the conclusion of each experimental trial, participants were asked to report any side 
112 
effects they might have encountered. 
113 
2.2 
Participants 
114 
The investigation involved a group of 13 male university-level rowers, with an average age of 22.07 
115 
years (standard deviation [SD] = 2.21), body mass of 77.66 kg (SD = 6.45), height of 182.14 cm (SD 
116 
= 7.11), body fat percentage of 11.23% (SD = 4.1), and a typical daily caffeine consumption of 303.62 
117 
mg (SD = 148.34). These individuals had accrued an average of 3.1 years (SD = 1) of rowing training 
118 
experience.  
119 
The required sample size was determined using G*Power software (version 3.1.9.4; Dusseldorf, 
120 
Germany), based on an analysis of variance (ANOVA) design incorporating repeated measures and 
121 
within-subjects factors. The calculation utilized an effect size (f) of 0.25, a significance level (alpha) 
122 
of 0.05, and a statistical power of 0.95 and r=0.85 with a single cohort of participants. 
123 
During their initial laboratory visit, participants underwent anthropometric assessments. Height and 
124 
body mass were recorded using a Seca stadiometer (Seca Deutschland, Hamburg, Germany), while 
125 
body fat percentage was evaluated with an InBody 770 body composition analyzer (InBody Co., 
126 

 
4 
Gangnam-Gu, Seoul, Korea). Following these measurements, participants were provided with detailed 
127 
study protocol information sheets and signed informed consent forms. 
128 
Eligibility for participation was restricted by specific exclusion criteria. Rowers were excluded from 
129 
the study if they presented with:  
130 
1. 
A medical condition that impaired their capacity to follow the study protocol;  
131 
2. 
Current use of prescription medications;  
132 
3. 
A confirmed allergy to mannitol or any other sweeteners;  
133 
4. 
A diagnosed sleep disorder; or  
134 
5. 
A physician9s recommendation to limit or avoid caffeine intake. 
135 
2.3 
Supplementation Protocol 
136 
Capsules, identical in appearance, were used to deliver the substances for all conditions. The dosages 
137 
of caffeine were individually calculated based on each participant9s weight. Placebo capsules were 
138 
filled with sugar alcohol (mannitol), not expected to have any further effects on performance (Flueck 
139 
et al., 2016). Caffeine capsules contained caffeine powder. In both caffeine and placebo experiments, 
140 
participants consumed 3 to 5 gelatin capsules, with the quantity of capsules standardized across all 
141 
subjects. Researchers and participants could not distinguish between placebo and caffeine capsules due 
142 
to equal color and size. At the end of each trial, participants were asked to indicate their assumption 
143 
regarding the type of capsules swallowed. The capsules were provided 60 minutes before each trial in 
144 
order to have sufficient time to ensure increasing blood caffeine levels (Pallares et al., 2013). The 
145 
supplementation groups were as follows; 3 mg/kg caffeine (low dose caffeine, LDC), 6 mg/kg caffeine 
146 
(moderate dose caffeine, MDC), 9 mg/kg caffeine (high dose caffeine, HDC) or placebo (PLA). Prior 
147 
to departing the laboratory, participants were queried regarding their perception of the treatment 
148 
administered, specifically whether they believed they had received no caffeine, a low dose, a moderate 
149 
dose, or a high dose. Furthermore, physical exhaustion and adverse effects (such as gastrointestinal 
150 
issues, tachycardia, muscle pain, or headaches) experienced 24 hours post-supplement administration 
151 
were documented via an online survey, following the methodology described by Pallares et al. (2013). 
152 
Owing to the physically demanding nature of the rowing time trial, the collection of side effect 
153 
responses was scheduled exclusively for 24 hours post-test. 
154 
2.4 
Diet and Caffeine Consumption Control 
155 
The research participants were instructed to abstain from alcohol consumption and vigorous physical 
156 
training for a 24-hour period preceding each experimental session. Throughout the duration of the 
157 
study, they were advised to refrain from using any dietary supplements. All participants were required 
158 
to maintain a detailed 24-hour dietary record on the day prior to each testing session, as well as a 
159 
weekly log of their caffeine intake. To ensure consistency in energy consumption and hydration status, 
160 
participants were directed to replicate their dietary intake, as documented in the food log, before every 
161 
trial. Daily caffeine consumption was quantified using a modified version of the questionnaire 
162 
developed by Bühler et al. (2014). Additionally, the caffeine content from various food and beverage 
163 
sources was incorporated to determine the total daily caffeine intake. Based on this evaluation, all 
164 
participants were classified as habitual moderate caffeine consumers, in accordance with the criteria 
165 
established by Filip et al. (2020). To simulate conditions reflective of real-world athletic environments, 
166 
as recommended by Tallis et al. (2021), participants were encouraged to maintain their usual daily 
167 
caffeine intake throughout the study. This approach was implemented to mitigate the potential impact 
168 
of caffeine withdrawal, as noted by Pickering and Kiely (2019).  
169 

 
5 
2.5 
Subjective Sleep Quality and Daytime Sleepiness Measurements 
170 
Participants were directed to maintain their normal sleep patterns both prior to and following the 
171 
experimental sessions. Each morning subsequent to a trial, they assessed their sleep quality utilizing a 
172 
validated numeric rating scale, with scores ranging from 0 (indicating "best possible sleep") to 10 
173 
(denoting "worst possible sleep") (Snyder et al., 2018). Furthermore, levels of daytime sleepiness were 
174 
evaluated in the afternoon through the application of the Karolinska Sleepiness Scale (Shahid et al., 
175 
2012), which employs a numeric range from 1 (representing "extremely alert") to 9 (signifying "very 
176 
sleepy"). 
177 
2.6 
2000-m Rowing Time Trial 
178 
The 2000 meters time trial was performed on the same rowing ergometer for each participant. The test-
179 
retest reliability of 2000m time trial on the Concept II rowing ergometer has previously been examined 
180 
previously with well-trained rowers and was reported with a coefûcient of variation (CV) of 0.6% 
181 
(Schabort et al., 1999). Each participant's warm-up routine was documented during the initial trial and 
182 
then consistently reproduced in the subsequent trials. The time to complete the time trial was recorded. 
183 
The stroke rate during the test was freely selectable by each subject and the drag factor settings of the 
184 
ergometers were adjusted to 140 as recommended by Amateur Rowing Association for heavyweight 
185 
men rowers (O9Neill & Skelton, 2004). During the 2k test, after a self-selected warmup, the athletes 
186 
were required to row 2000m in the least time possible. This test is a standard criterion used for national 
187 
team selection purposes in many countries (Hahn et al., 2000; Pripstein et al., 1999) and was performed 
188 
routinely by all rowers in this study. 
189 
2.7 
Statistical Analysis  
190 
The normality of the dataset was initially assessed using the Shapiro-Wilk test. Following this, the 
191 
sphericity assumption was evaluated with Mauchly9s test, and the Greenhouse-Geisser correction was 
192 
implemented whenever violations of sphericity were evident. To examine differences in test 
193 
completion time, heart rate, power output, and subjective sleep and daytime sleepiness parameters 
194 
across the full duration of the testing period, a repeated measures analysis of variance (ANOVA) was 
195 
employed. The partial eta square (·p2) was utilized to assess the effect size, which was classified as 
196 
small (0.1030.24), moderate (0.2530.39), or large (g0.40). Where significant effects emerged, 
197 
Bonferroni post hoc paired comparisons were conducted to identify specific differences between 
198 
conditions. Cohen9s d effect sizes for repeated measures, reported alongside their 95% confidence 
199 
intervals (95% CI). These effect sizes were interpreted according to the following benchmarks: values 
200 
less than 0.20 were deemed trivial, those ranging from 0.20 to 0.49 were classified as small, 0.50 to 
201 
0.79 as moderate, and values of 0.80 or higher as large (Cohen, 1992). Statistical computations were 
202 
executed using SPSS software (version 30; IBM Corp., Armonk, New York, USA), while data 
203 
visualizations were created with GraphPad PRISM Software (Version 10.4, GraphPad Inc., San Diego, 
204 
CA, USA). Additionally, a chi-square analysis was employed to investigate differences in the 
205 
prevalence of side effects associated with varying caffeine doses among athletes 24 hours post-
206 
administration. 
207 
3 
Results 
208 
Effects of different supplementations on rowing ergometer time trial time have been demonstrated in 
209 
figure 1. 
210 

 
6 
 
211 
There was a significant difference in rowing time trial performance between conditions (p=0.001, 
212 
·p2=0.45). Bonferroni post-hoc analysis showed time trial completed faster in HDC condition 
213 
(403.30±10.66 seconds) than PLA (407.62±10.77 seconds, p=0.013; 95% Confidence Interval 
214 
(CI)=0.80737.793; d=0.40). Time trial performance was significantly better in MDC (404.29±10.57 
215 
seconds) group compared to PLA (407.62±10.77, p=0.039; 95% CI=0.13636.356; d=0.30). In addition, 
216 
the results were significantly better for HDC group (403.30±10.66 seconds) compared to LDC group 
217 
(405.68±11.07 seconds, p=0.024; 95% CI=-4.590 3 -0.271; d=0.23). Despite non-significance, mean 
218 
values of LDC (405.68±11.07 seconds, p=0.197; d=0.13) were superior to PLA (407.62±10.77 
219 
seconds) which could be practically relevant in the field settings. 
220 
Mean power output during Time trial has been provided in Figure 2 
221 
380
390
400
410
420
430
Seconds
PLA
HDC
MDC
LDC
Figure 1.
2000 m. rowing ergometer Time Trial completion time.
PLA=Placebo, HDC=High Dose Caffeine, MDC=Moderate Dose Caffeine,
LDC=Low Dose Caffeine, *=p<0.05
7
7
7

 
7 
 
222 
There was a significant difference in time trial mean outputs between conditions (p=0.001, ·p2=0.47). 
223 
Bonferroni post-hoc analysis showed higher power outputs for HDC (342.77±26.67 watts) compared 
224 
to PLA (332.1±25.9 watts, p=0.009; 95% CI=-19.118 3 -2.513; d=0.41). The results were significantly 
225 
better for HDC group (342.77±26.67 watts) compared to LDC group (336.88±27.05 watts, p=0.028; 
226 
95% CI=0.537311.263; d=0.23). In addition, higher power outputs for MDC (340.24±26.24 watts) 
227 
were detected compared to PLA (332.1±25.9 watts, p=0.027; 95% CI=-15.782 3 -0.788; d=0.31).  
228 
Average heart rate values obtained during time trial have been presented in figure 3. 
229 
280
300
320
340
360
380
400
Watts
Figure 2
Mean power output during 2000 m. rowing ergometer Time Trial.
PLA=Placebo, HDC=High Dose Caffeine, MDC=Moderate Dose Caffeine,
LDC=Low Dose Caffeine, *=p<0.05, **=p<0.01.
77
7
7
PLA
HDC
MDC
LDC

 
8 
 
230 
There was a significant difference in heart rate values between conditions (p=0.001, ·p2=0.56). 
231 
Bonferroni post-hoc analysis showed higher hear rate for HDC (185.54±4.43 beats per minute [bpm]) 
232 
compared to PLA (183.08±3.88 bpm, p=0.002; 95% CI=-3.228 3 -1.695; d=0.72). In addition, the 
233 
results were significantly different between MDC group (184.46±4.34 bpm) and PLA group 
234 
(183.08±3.88 bpm, p=0.005; 95% CI=-2.365 3 -0.405; d=0.40).  
235 
Subjective sleep quality and Karolinska daytime sleepiness scale values has been provided in Figure 
236 
4.  
237 
There was a significant difference in sleep variables between conditions (p=0.002, ·p2=0.42). 
238 
Bonferroni post-hoc analysis showed worse sleep quality for HDC (4±0.71) compared to PLA (3±0.77, 
239 
p=0.005; 95% CI=-1.714 3 -0.286; d=1.5). Significantly worse sleep quality scores have been reported 
240 
in MDC (3.85±0.69) trial than PLA (3±0.77, p=0.032; 95% CI=-1.632 3 -0.060; d=1.3). On the other 
241 
hand, participants in LDC trial reported worse sleep quality despite non-significance, values were close 
242 
to significance (p=0.082) and may have a practical effect during practice. significant difference in sleep 
243 
variables between conditions (p=0.002, ·p2=0.42). 
244 
Significant differences in daytime sleepiness have been reported between conditions (p=0.002, 
245 
·p2=0.37). Daytime sleepiness values reported in HDC trial (4.31±0.95) were significantly higher than 
246 
PLA (3.23±0.73, p=0.009, 95% CI=-1.911 3 -0.243; d=1.4). Similarly, daytime sleepiness values 
247 
reported during MDC trial (4.23±0.73) were significantly higher than PLA (3.23±0.73, p=0.005, 95% 
248 
CI=-1.714 3 -0.286; d=1.3). 
249 

 
9 
 
250 
Reported side effects after 24 hours post-test was presented in Table 1. 
251 
Table 1. The incidence of side effects reported by participants (n = 13) 
252 
Variables 
PLA 
LDC 
MDC       HDC 
Increased urine output 
0 
1 
2             5*# 
Muscle soreness 
1 
1 
2             2 
Headache 
0 
0 
1               5*#  
Tachycardia  
0 
0 
2             4*# 
Increased vigor/activeness 
0 
0 
1             2*# 
Anxiety or nervousness 
0 
0 
1              4*#  
Gastrointestinal problems 
0 
0 
1               6*#  
Insomnia 
0 
2 
5             7*# 
PLA=Placebo, HDC=High Dose Caffeine, MDC=Moderate Dose Caffeine, LDC=Low Dose Caffeine,  
253 
*=p<0.05 vs. PLA, #=p<0.05 vs. LDC,  =p<0.05 vs. MDC. 
254 
Significantly higher prevalence of adverse side effects such as headache, gastrointestinal problems, 
255 
increased urine output and insomnia were reported after HDC condition compared to all other 
256 
conditions. Just three participants accurately discerned two out of the four experimental conditions, 
257 
while another three correctly identified only one condition. Collectively, these results suggest that the 
258 
randomization process was effective. Only one participant out of thirteen accurately identified the full 
259 
set of experimental conditions, indicating that the blinding procedure was successful. 
260 
4 
Discussion 
261 
0
2
4
6
Figure 4
Subjective sleep quality and daytime sleepiness values.
Karolinska
PLA=Placebo, HDC=High Dose Caffeine, MDC=Moderate Dose Caffeine,
LDC=Low Dose Caffeine, *=p<0.05.
SSQ
Scale Scores
PLA
HDC
MDC
LDC
7
7
7
7

 
10 
The current study presents an experimental design that examines the effects of different doses of 
262 
caffeine (low: LDC, medium: MDC, high: HDC) taken in the evening on timed rowing performance, 
263 
cardiovascular responses, sleep quality, and daytime alertness. The main findings of the study showed 
264 
that MDC and HDC in particular produced ergogenic effects in terms of performance, but these effects 
265 
were accompanied by significant sleep disruptions and side effects with increasing doses. 
266 
When previous studies were examined, it was reported that they performed better in the afternoon 
267 
(17:00) compared to the morning session in the 5-meter multiple shuttles run test (Souissi et al., 2019). 
268 
In another study, when the effect of time of day on repeated sprint performance was examined, higher 
269 
muscle strength was reported during the afternoon hours or early evening compared to the morning 
270 
hours (Aloui et al., 2013). However, these studies did not evaluate the athletes' fixed-distance time trial 
271 
performances. On the other hand, in a study conducted specifically on rowers, in which performances 
272 
on the 2000m rowing ergometer in the morning and evening were evaluated, it was reported that they 
273 
performed an average of 2.4 seconds faster in the morning training compared to the evening, regardless 
274 
of chronotype (Brown et al., 2008). Considering this research, it is understandable that rowers need 
275 
supplements to optimize their performance in the evening training. In the current study where the test 
276 
sessions were conducted in the evening hours, the 2000 m rowing ergometer completion time was 
277 
significantly shorter in the HDC (9 mg/kg) and MDC (6 mg/kg) conditions compared to PLA, 
278 
indicating a dose-dependent performance increase. Although statistical significance was not obtained 
279 
in the LDC (3 mg/kg) condition, the decrease in mean time may be important in practical applications. 
280 
Even 1% changes in mean speeds in intense Olympic endurance events lasting approximately 45 
281 
seconds to 8 minutes are sufficient to affect the medal rankings of athletes (Christensen et al., 2017). 
282 
Especially in rowing, which is a race against time, gains of seconds may be decisive. The findings of 
283 
the current study are consistent with previous studies on the effects of caffeine on short-term high-
284 
intensity performance (Anderson et al., 2000; Bruce et al., 2000; Davis & Green, 2009; Southward et 
285 
al., 2018). Similar to our findings, Bruce et al. (2000) in an experimental study in which diet and 
286 
training were well controlled, it was observed that caffeine (6 mg/kg or 9 mg/kg) consumed before a 
287 
short-term high-intensity endurance test provided a significant increase in performance. In another 
288 
study, 3 mg/kg caffeine intake in the evening did not significantly improve athletes' 100m swimming 
289 
time trial performances (Newbury et al., 2022). Doherty et al. (2004) reported that caffeine had a more 
290 
limited ergogenic effect in intense and short-term exercises compared to long-term endurance 
291 
exercises. Accordingly, it can be thought that higher doses are required for the ergogenic effect of 
292 
caffeine in short-distance time trial performances to be seen. In addition, the average daily caffeine 
293 
consumption of the participants in the current study was 303.62 mg (SD = 148.34). Considering the 
294 
possibility of developing tolerance to caffeine, the amount consumed by the participants in the low-
295 
dose caffeine condition is quite low compared to their daily habits. This may explain why the 
296 
improvement in time trial performance observed in the high and medium dose caffeine conditions was 
297 
more pronounced compared to the low dose condition. 
298 
Power output is usually the primary performance measure for ergometer tests. It is essential to establish 
299 
a reliable rowing performance test to test the effectiveness of an intervention on a rower's power 
300 
generation ability (Soper & Hume, 2004). The mean percentage standard error (%SEM) for mean 
301 
power between repeated 2000 m performances on the Concept II ergometer has been reported as 2.0% 
302 
(Schabort et al., 1999). In other words, it can be said that the 2.0% difference in mean power in repeated 
303 
tests is due to natural measurement variability. In the present study, the 3.2% increase in mean power 
304 
obtained in HDC and PLA conditions is above the 2.0% natural measurement variability. This supports 
305 
the possibility that the observed difference is due to an ergogenic effect. There is also a significant 
306 
difference between MDC and PLA. MDC supplementation resulted in a 2.45% increase in average 
307 
strength compared to PLA. Similar to the HDC condition, this increase over the natural measurement 
308 

 
11 
variability supports the significant effect of the MDC intervention. The findings of the present study 
309 
are consistent with the literature. Chen et al. (2024) in their meta-analysis on time trial performance of 
310 
cyclists, supported that moderate dose caffeine intake led to significant improvements in performance 
311 
variables including average power output. In addition, a significant difference was found between the 
312 
mean power outputs observed between HDC and LDC conditions in favor of HDC. The 1.75% 
313 
difference between the mean power outputs suggests that the effect of high-dose caffeine intake on 
314 
performance is dose-dependent, similar to our findings in time trial performance. Although the 
315 
significant difference between HDC and LDC is within the natural measurement variability limit of 
316 
the test, it has practical importance in time trial rowing. The findings of our research are consistent 
317 
with the literature (Anderson et al., 2000; Bruce et al., 2000; Wiles et al., 2006). According to the 
318 
analyses of the current study, the improvements in power observed after caffeine ingestion are similar 
319 
to those reported for elite rowers by Bruce et al. (2000) (mean power increase of 2.7% in the 2000 m 
320 
rowing test). In another study evaluating the 100m time trial performance of swimmers, 5 mg/kg 
321 
caffeine intake shortened both the completion time and increased the average power production (3.6%) 
322 
(Wiles et al., 2006). Finally, in our study, participants were classified as habitual moderate caffeine 
323 
consumers (303.62 mg [SD = 148.34]) (Filip et al., 2020). LDC intake, which corresponds to below 
324 
the average daily caffeine consumption of the participants, did not have a significant effect on average 
325 
power compared to PLA. This may be due to the dose being insufficient to increase performance. 
326 
Considering the time trial times and power outputs, it can be said that caffeine intake improves rowing 
327 
performance. These results are consistent with the findings of a meta-analysis examining the effect of 
328 
caffeine on time trial performance in cyclists and using completion time and average power output as 
329 
performance measures (Chen et al., 2024). Meta-analysis has shown that moderate doses of caffeine 
330 
(4-6 mg/kg), which is determined to be the optimal dose range, significantly improve time trial 
331 
performance in cyclists, whereas low doses (1-3 mg/kg) do not provide the same improvement. 
332 
In the current study, when HDC and MDC caffeine intakes were compared with PLA, significant 
333 
differences in mean heart rate were associated with small and moderate increases. It is known that the 
334 
autonomic nervous system, which is managed by the central nervous system, plays an important role 
335 
in controlling heart rate (Ginty et al., 2017; Quinlan et al., 1997). Considering the effect of caffeine on 
336 
the central nervous system, it is reasonable that it plays a role in changes in heart rate (Benjamim et 
337 
al., 2020). Accordingly, the heart rate increases observed in HDC and MDC conditions support the 
338 
known sympathetic stimulating effect of caffeine. This finding shows that the load on the 
339 
cardiovascular system increases simultaneously with the increase in performance. Although an 
340 
increase in heart rate during exercise is a physiologically expected result, this may lead to tolerance 
341 
problems in some individuals during submaximal exercises. Contrary to the current research findings, 
342 
Bruce et al. (2000) did not observe a significant difference in heart rate during exercise after 6 and 9 
343 
mg/kg caffeine intake. The contradictory physiological responses to caffeine and exercise may be due 
344 
to multiple factors, including timing of measurement, individual metabolic differences, and 
345 
consumption habits (Aguilar-Navarro et al., 2019; Pickering & Kiely, 2019). In the relevant study, no 
346 
information was provided regarding the caffeine use history of the sample group. In addition, although 
347 
it was stated that blood samples were taken 45 minutes after caffeine intake and a warm-up protocol 
348 
was applied, it was not clearly stated when the main performance test was started. 
349 
Despite the potential of caffeine to enhance performance, sleep deprivation, which poses a risk of 
350 
performance deficiencies, is among its known side effects (Hindmarch et al., 2000; Youngstedt et al., 
351 
2000). Drake et al. (2013) stated that even 400 mg of caffeine taken 6 hours before bedtime 
352 
significantly reduced sleep quality. Furthermore, it is known that caffeine (200 mg, 3+3 mg/kg, 6 
353 
mg/kg), especially in the afternoon, both before and without exercise, causes deterioration in sleep-
354 
related parameters (Drapeau et al., 2006; Miller et al., 2014; Ramos-Campo et al., 2019). The findings 
355 

 
12 
of the current study, in which the test sessions started between 19:00 and 20:00, support the literature. 
356 
Caffeine consumption was found to have a significantly deteriorating effect on sleep quality in the 
357 
following night in HDC and MDC conditions compared to PLA. Although the pharmacokinetics of 
358 
caffeine vary between individuals (Temple et al., 2017), it generally has a half-life ranging from 2 to 
359 
10 hours (Snel & Lorist, 2011). Although the plasma concentration of caffeine decreases over time, it 
360 
is likely that it will continue to have a stimulating effect until it is completely eliminated, thus 
361 
negatively affecting sleep quality. Although sleep quality was assessed only via a scale in the current 
362 
study, Ali et al. (2015) showed that VO¢, RER, and heart rate values measured during sleep remained 
363 
high after caffeine intake. These findings support the idea that caffeine may impair sleep quality by 
364 
maintaining metabolic stimulation even during sleep. In addition, although the deterioration in sleep 
365 
quality in LDC did not reach statistical significance in our study, the trend was negative. Although 
366 
caffeine taken in the evening can disrupt all types of sleep parameters through 6-sulfatoxymelatonin 
367 
(the main metabolite of melatonin) (Shilo et al., 2002) and adrenaline and noradrenaline stimulation in 
368 
the adrenal medulla (Sawyer et al., 1982; Zhang et al., 2020), the majority of sleep disruptions are 
369 
thought to be dose-dependent (Robillard et al., 2015). Karacan et al. (1976) reported that among the 
370 
caffeine doses given 30 minutes before bedtime, caffeine equivalent to 4 cups of coffee disrupted total 
371 
sleep time, emphasizing that the effect was dose-related. In parallel with our findings, a study 
372 
conducted with highly trained judokas reported that low-dose (3 mg/kg) caffeine intake administered 
373 
before evening training did not cause a significant deterioration in objective sleep quality (Filip-
374 
Stachnik, 2022). In contrast, Miller et al. (2014) concluded that 3 mg/kg caffeine caused significant 
375 
sleep disruption. However, in this study, participants did not do any exercise after caffeine intake. It is 
376 
known that acute exercise improves both sleep quality and sleep latency (Driver & Taylor, 2000; 
377 
Youngstedt, 2005). In the current study, the negative effect of low-dose caffeine on sleep disruption 
378 
may have been inhibited thanks to this known benefit of exercise. In addition, although not significant, 
379 
the tendency of low-dose caffeine to impair sleep quality indicates that individual sensitivity and 
380 
tolerance levels should be taken into account (Aguilar-Navarro et al., 2019). Although low-dose 
381 
caffeine did not provide a significant improvement in 100m time trial performances in swimmers, the 
382 
fact that effect size analyses showed that it may negatively affect sleep quality in some parameters 
383 
(total sleep time -32 minutes; time to fall asleep +14 minutes) suggests that individual conditions 
384 
should be evaluated well. 
385 
When daytime sleepiness data are examined, it is remarkable that there is a significant increase in HDC 
386 
and MDC conditions compared to PLA. This observed increase may be associated with the decrease 
387 
in the quality of nighttime sleep at the same doses. Laboratory studies show that an average decrease 
388 
of 90 minutes in nighttime sleep can lead to a decrease in objective alertness the next day by 
389 
approximately one-third (Bonnet & Arand, 2010). Moreover, sleep deprivation causes significant 
390 
decreases in cognitive functions such as cognitive and psychomotor reaction speed in the following 
391 
days (Goel et al., 2009). This may be a limiting factor in terms of the sustainability of performance. It 
392 
has been reported that even one night of disrupted sleep negatively affects performance in the next 
393 
competition or training (Oliver et al., 2009; Skein et al., 2011). Therefore, it is important to be aware 
394 
of the potential risks of caffeine use, which is well known to have negative effects on sleep. 
395 
Lastly, when evaluated in terms of side effects, the current study reported that after the HDC condition, 
396 
effects such as headache, gastrointestinal problems, frequent urination, and insomnia increased 
397 
significantly. Similar to the findings of our study related to LDC, in the study of Newbury et al. (2022), 
398 
no significant difference was found in self-reported physical or psychological side effects of 3 mg/kg 
399 
caffeine compared to placebo. 
400 

 
13 
The current study has several strengths. It is one of the few studies that examines the effects of pre-
401 
exercise caffeine use on both performance and sleep. Furthermore, it is one of the studies that can lead 
402 
to the determination of the most appropriate supplementation amount by comparing different doses. 
403 
Reliable and reproducible data were provided through the analyses. However, certain limitations 
404 
should be acknowledged. Since plasma caffeine concentrations were not measured, it was not possible 
405 
to determine to what extent the observed sleep disturbances were related to this variable. Additionally, 
406 
sleep parameters were assessed only through self-reporting. However, individual differences could not 
407 
be taken into account because the participants' habitual sleep patterns were not monitored. Finally, the 
408 
sample group consisting of trained individuals with moderate caffeine consumption habits limits its 
409 
generalizability to different training and caffeine sensitivity levels. 
410 
5 
Conclusion 
411 
In conclusion, the effects of different doses of caffeine taken before exercise in the evening on rowing 
412 
performance against time, cardiovascular responses, sleep quality and daytime alertness were 
413 
investigated in the present study. The caffeine dose consumed is a determining factor on the time trial 
414 
performance in the 2000m rowing ergometer. The research findings showed that high and medium 
415 
doses of caffeine supplementation caused performance improvement compared to placebo in the time 
416 
trial test, while low dose caffeine dose did not show a significant difference. However, the observed 
417 
improvement was close to significance. Similarly, it was seen that high and medium doses provided a 
418 
significant improvement compared to placebo in the mean power output findings. In addition, high 
419 
dose caffeine supplementation caused a significant increase compared to low dose. When the mean 
420 
heart rates monitored during the test were examined, a significant difference was observed only 
421 
between high dose and placebo. Finally, it was observed that high and medium doses significantly 
422 
impaired sleep quality compared to placebo in the night sleep following caffeine use. Low dose 
423 
caffeine, on the other hand, did not show a significant result despite its tendency to disrupt sleep. It has 
424 
been proven that athletes' sleepiness the day after caffeine use was significantly higher in high and 
425 
medium dose conditions. 
426 
In summary, it is shown that high-dose caffeine consumption in the evening hours leads to an 
427 
improvement in performance, but also brings about negative situations such as sleep problems and 
428 
physical side effects. Athletes should consider the advantages and disadvantages when choosing a 
429 
supplemental dose. When the research findings are examined, the MDC condition stands out as a 
430 
balanced option in practical applications due to its limited negative effects on sleep, low side effect 
431 
reports and performance improvement. Future studies can examine the effects of different doses of 
432 
caffeine taken at different times of the day on sleep variables. In addition, in our study, sleep variables 
433 
were assessed only with subjective scales. Studies using actigraphy or polysomnography can be 
434 
designed. Although the sleepiness of the athletes the next day was evaluated, how caffeine-related sleep 
435 
deprivation affects performance, especially in athletes competing on consecutive days, can be an 
436 
interesting research topic. Finally, genetic metabolism differences (such as CYP1A2 genotype) of the 
437 
participants can be analyzed to determine their sensitivity to caffeine in terms of sleep and contribute 
438 
to making comments at an individual level. 
439 
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