Operation early-bird: Investigating altered light exposure in military barracks on sleep and performance-a...

The manipulation of light exposure in the evening has been shown to modulate sleep, and may be beneficial in a military setting where sleep is reported to be problematic. This study investigated the efficacy of low-temperature lighting on objective sleep measures and physical performance in military trainees. Sixty-four officer-trainees (52 male/12 female, mean ± SD age: 25 ± 5 years) wore wrist-actigraphs for 6 weeks during military training to quantify sleep metrics. Trainee 2.4-km run time and upper-body muscular-endurance were assessed before and after the training course. Participants were randomly assigned to either: low-temperature lighting (LOW, n = 19), standard-temperature lighting with a placebo "sleep-enhancing" device (PLA, n = 17), or standard-temperature lighting (CON, n = 28) groups in their military barracks for the duration of the course. Repeated-measures ANOVAs were run to identify significant differences with post hoc analyses and effect size calculations performed where indicated. No significant interaction effect was observed for the sleep metrics; however, there was a significant effect of time for average sleep duration, and small benefits of LOW when compared with CON (d = 0.41-0.44). A significant interaction was observed for the 2.4-km run, with the improvement in LOW (Δ92.3 s) associated with a large improvement when compared with CON (Δ35.9 s; p = 0.003; d = 0.95 ± 0.60), but not PLA (Δ68.6 s). Similarly, curl-up improvement resulted in a moderate effect in favour of LOW (Δ14 repetitions) compared with CON (Δ6; p = 0.063; d = 0.68 ± 0.72). Chronic exposure to low-temperature lighting was associated with benefits to aerobic fitness across a 6-week training period, with minimal effects on sleep measures.