Discussion
SE, sleep complaints
The main findings of this study are that 7 days of CP supplementation; 1) reduced awakenings, as measured objectively by PSG, and subjectively via sleep diaries, and 2) improved cognitive performance, as measured by the Stroop test. This is the first study to show that CP supplementation may reduce sleep fragmentation in athletic males and suggests CP could be used as a non-pharmacological strategy to enhance sleep quality in athletes, and potentially other populations with sleep complaints.We found no effect of CP supplementation on our primary outcomes (SOL and SE) or sleep architecture, but did find that CP reduced awakenings, as measured objectively by PSG and subjectively with sleep diaries. Our PSG data showed that another key indicator of sleep fragmentation, WASO, was ~ 5.5 min lower with CP (Table The mechanisms by which CP reduced awakenings are unclear. The main mechanism by which we hypothesized CP would modulate sleep quality and architecture was related to its high glycine content and effect on circadian rhythms. Indeed, Kawai et al., [While CP had no effect on tests of RT or short-term memory, it did increase the proportion of correct responses on the baseline Stroop test the morning after the main PSG trial night. Our results are in partial agreement with previous studies examining the effects of glycine on sleep and subsequent cognitive performance. Yamadera and colleagues [It is unclear why CP only influenced baseline Stroop test performance; previous studies do not suggest that the Stroop test, which chiefly measures the ability to inhibit cognitive interference, is more sensitive to changes in sleep quality than tests of RT or other aspects of cognition [This study has many strengths. Firstly, sleep was assessed objectively using the gold standard PSG method. Furthermore, we familiarised participants to wearing the PSG equipment with a pre-intervention familiarisation week and then mitigated any bias introduced with a first night effect [In conclusion, our findings demonstrate that 7 days of CP supplementation (15 g/day) reduced nocturnal awakenings and improved Stroop test cognitive performance. While these findings should be interpreted with caution until replicated by future studies, they suggest CP supplements could enhance sleep quality in athletic populations with sleep complaints. As athletes often report lower sleep quality than the general population, especially after late night competitions, strategic intake of CP could mitigate the potentially deleterious effects of poor sleep on psychomotor performance.