## 4 Discussion
had no significant impact on objective sleep measurements. However, from a practical point of view, it is important to use caffeine doses that simultaneously improve exercise performance without negatively impacting sleep and recovery. The studies mentioned above, analyzing sleep following ingestion of 6 mg/kg of caffeine, presented conflicting results [30; 31], showing significant improvement in the trials for cycling [30] and counter-movement jumping [31], but no effect on 800 m running time [31]. Taking into account the fact that the optimal caffeine dose may differ depending on the caffeine source [57], performance test [7; 24; 25; 28; 58; 59; 60], and the individual [61], further dose-response studies which include sleep assessments are needed. While the results of the current study found a significant decrease only in self-reported sleep quality, the \"easy of falling asleep\" responses also favored the placebo condition, showing a large effect size (4.3 \(\pm\) 0.9 vs. 3.1 \(\pm\) 1.7; ES: \(-\)0.88). Similarly, by administering 6 mg/kg of caffeine before evening training, a previous study found significant differences in subjective quality of sleep and sleep latency [31; 62]. Such studies emphasize the fact that objective and subjective assessments of sleep quality may relate to different parameters [63], and it seems that these parameters might be particularly sensitive to caffeine ingestion. Additionally, the results of the current study revealed no significant changes in self-reported recovery-stress state on the morning following the experimental sessions. However, it is worth noting that all sleep and recovery parameters were affected by the caffeine condition (ES = 0.09-1.09). Even if almost all of the outcomes did not reach statistical significance, it cannot be assumed that the observed sleep interruptions had no critical health consequences. It is worth highlighting the fact that most adults require seven to nine hours of sleep per night [64], and that athletes may need more due to the high physical and mental demands under which they are placed [29]. Interestingly, in the current study, the sleep duration was shorter than the recommended seven hours during the caffeine condition. Taking into account the fact that elite athletes are particularly susceptible to sleep disturbances, characterized by chronic short sleep (\(\lesssim\)7 h/night) and poor sleep quality (e.g., sleep fragmentation) [29], such observed decreases might contribute to negative health and compromise an athlete's capacity to train effectively and compete optimally. Further, it is possible that with the continuous use of caffeinated supplements before evening training sessions, the sleep and recovery parameters might be significantly disturbed over a longer period of time. It should be taken into consideration that athletes in the current study were habitual caffeine users. In the present investigation, the mean daily level of caffeine consumption was similar to the single caffeine dose used before the training sessions (2.9 mg/kg vs. 3 mg/kg). Habitual caffeine users may develop tolerance to certain physiological effects of caffeine [51], and thus, a diminished effect on sleep parameters might result from the athletes' daily caffeine consumption. For this reason, it is likely that the very low frequency of side effects and lack of changes in heart rate during the judo training session or the night after the experimental trials were found in this study, results similar to those observed in previous studies [24; 30; 52]. Taking into account the fact that caffeine is widely and frequently consumed through coffee, tea, and commercially available soft and energy drinks [65], athletes should control their daily habitual caffeine intake if they are considering the use of caffeinated sports supplements. It is worth noting that the current recommendations for athletes only emphasize avoiding high caffeine doses (i.e., \(\geq\)9 mg/kg) [66] or stimulants too close to bedtime [29].