## 2 Materials and Methods
### Experimental Design  
To investigate the effect of caffeine on sleep quality and recovery-stress state, the athletes underwent a randomized, double-blind, placebo-controlled crossover experiment in which each athlete acted as their own control. The blinding and randomization of the experimental sessions were conducted by a member of the research team (using www.randomization.com (accessed on 3 August 2022)) who was not directly involved in data collection; thus, after assignment to interventions, both athletes and researchers were blinded to the trials. The study was conducted immediately after the competitive season, and all athletes participated in judo training sessions five times a week from 7:00 to 8:30 PM (Monday to Friday) and in strength and conditioning training two times a week from 10:00 to 11:30 AM (Tuesday and Thursday) during the study period. Each athlete took part in a familiarization session and two identical experimental trials that included the ingestion of (i) a placebo and (ii) 3 mg/kg of caffeine, administered 60 min before the onset of the exercise protocol. Capsules with caffeine (100% purity) and placebo (cellulose) were prepared by the manufacturer (Olimp Laboratories, Debica, Poland). The familiarization session and the trials were conducted on three consecutive Mondays at 7:00 PM during the athletes' regular training sessions. During the familiarization session, the athletes' body composition was evaluated using bioimpedance (model 370, InBody, Biospace Co., Seoul, Korea), and they were informed about the experimental procedures. After the session, the athletes received GENEActiv accelerometers (Activinsights, Cambridge, UK) and were asked to wear them on their non-dominant wrist for the study period.  
During each experimental session, athletes performed an identical warm-up, simulating a pre-competition warm-up, and completed a standardized randori training session under controlled ambient conditions. The randori session consisted of 3 fighting bouts of 4 min, with 10 min of rest between fighting bouts. During randori, the athletes fought with opponents in the same weight category and with a similar judo level. The athletes were instructed to fight to score the most points or win by ippon, yet the bout was continued regardless of the score. The heart rate during the training sessions was measured using a heart rate monitor (Polar H10, Finland). Immediately after the training session, rating of perceived exertion (RPE; using the 1-10-point Borg scale [41]) was evaluated.  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **Variable (Units)** & **Men (_n = 5_)** & **Women (_n = 4_)** \\ \hline Age (years) & \(24\pm 5\) & \(20\pm 1\) \\ \hline Body mass (kg) & \(73.4\pm 11.6\) & \(70.1\pm 15.0\) \\ \hline Height (cm) & \(175\pm 5\) & \(169\pm 2\) \\ \hline Body Fat (\%) & \(12.6\pm 5.7\) & \(18.8\pm 9.5\) \\ \hline Judo training experience (years) & \(15\pm 6\) & \(12\pm 3\) \\ \hline Habitual caffeine intake (mg/kg/day) (mg/kg/day) & \(2.6\pm 1.9\) & \(3.0\pm 2.9\) \\ \hline Data reported as mean \(\pm\) standard deviation. & \\ \end{tabular}
\end{table}
Table 1: Main participants’ characteristics.