## 1 Introduction  
Caffeine ranks as one of the world's most commonly used psychoactive substances [1], and is widely consumed by athletes in all sports disciplines [2; 3; 4]. Interestingly, more than \(75\%\) of athletes consume caffeine before or during competitive events [2; 3; 4]. Strong evidence supports the idea that caffeine ingestion at a dose of 3-6 mg per kilogram (mg/kg) of body mass improves performance in a broad range of exercise tasks [2]. Indeed, several previous investigations have confirmed the positive impact of acute caffeine intake on endurance [5], anaerobic [6], and resistance performance [7], as well as performance in sports that require a substantial contribution from different energy metabolisms like combat [8; 9] or team sports [10; 11]. Although caffeine has ergogenic potential [12], its ingestion can also produce troubling side effects such as headache, nausea, insomnia, or anxiety [13]. Consequently, through caffeine intake, athletes' recovery and well-being might be limited, and this may impact their performance over the following days [14]. A recent meta-analysis that examined side effects associated with caffeine supplementation in athletes found that the incidence of side effects after consumption of low and moderate doses of caffeine (i.e., \(\leq\)6 mg/kg) could be as high as 34% [15]. It is worth noting that the occurrence of tachycardia/heart palpitations and sleep problems were the most commonly reported side effects [15]. Because growing evidence suggests a detrimental effect of sleep deprivation on sport-specific and exercise performance, athletes should avoid situations that pose a risk to their sleep quality [16; 17]. Although several previous investigations have shown the negative impact of caffeine on sleep in the general population [18; 19; 20; 21], less attention has been paid to analyzing sleep measures among athletes [15]. Even though some previous studies examined the occurrence of insomnia or sleep problems [9; 22; 23; 24; 25; 26; 27; 28], polysomnography or actigraphy, typically used for sleep monitoring in athletes [29], was not commonly used [15]. To date, only two studies have explored the acute impact of caffeine intake on sleep quality in athletes using these objective methods [30; 31]. In the study of [PERSON] et al. [30] (using polysomnography), 6 mg/kg of caffeine ingested before late-afternoon cycling exercises (i.e., ~5:00 PM) increased sleep onset latency and decreased sleep efficiency, rapid eye movement sleep, and total sleep time. Similarly, in a study conducted by [PERSON] et al. [31], 6 mg/kg of caffeine administered before an 800 m running test performed in the evening (i.e., 8 PM) impaired sleep efficiency, actual wake time, and the number of awakenings measured by actigraphy. Moreover, the subjective sleep parameters reported by the runners in the Karolinska Sleep Questionnaire, including \"sleep quality,\" \"calm sleep,\" \"ease of falling asleep,\" and \"feeling refreshed after waking,\" were interrupted at night after the caffeine intake [31]. Two additional studies have explored the relationship between individual caffeine ingestion on the sleep quality of rugby athletes [32; 33], showing a negative [33] and neutral [32] impact of evening pre-game caffeine ingestion on sleep-related measures. In light of these findings, it seems that the use of caffeine for training purposes is a relevant issue worth careful consideration by athletes concerned with sleep hygiene. Thus, verifying the impact of different caffeine doses on sleep and recovery seems especially important for athletes who use caffeine before evening training or competitions, or in sports where final scores are decided through competition over consecutive days [14]. From a practical standpoint, using a dose that simultaneously improves sports performance and has no adverse effect on sleep and recovery might be useful.