The Effect of Muscle-Damaging Exercise in the Heat on Sleep

Sleep is negatively affected by muscle-damaging exercise, but it remains unknown whether hot environmental conditions further affect sleep.

Sleep quality would decline, but sleep quantity would be longer after experiencing both heat exposure and EIMD.

Crossover study.

Level 3.

In a randomized, counterbalanced order, 10 healthy males (mean ± SD: age, 23 ± 3 years; body mass, 78.7 ± 11.5 kg; height, 176.9 ± 5 cm; lactate threshold [LT], 9.7 ± 1.0 km h^-1) performed 30 min of downhill running (DHR) at -10% gradient at the LT in control (ambient temperature [T_amb], 20°C; relative humidity [RH], 20%) and hot conditions (T_amb, 35°C; RH, 40%). Seven days later, participants performed a flat 45-min run in the heat at LT. Sleep parameters were collected from a wearable device the night after DHR, the following 6 nights, and the night of the 45-min flat run. Differences in sleep parameters between conditions following DHR, the subsequent 6 nights, and the night of the 45-min run were analyzed using repeated measures analysis of variance (ANOVA).

Following DHR, total sleep time (6.7 ± 0.7 h vs 5.2 ± 1.8 h; P = 0.040), rapid eye movement (REM; 1.7 ± 0.6 h vs 1.2 ± 0.7 h; P = 0.046), and slow-wave sleep (SWS; 1.6 ± 0.4 h vs 1.2 ± 0.5 h; P = 0.015) were greater in the hot condition. However, REM% and SWS% did not differ (P > 0.05), indicating increases reflected longer sleep duration rather than altered architecture. Sleep efficiency, light sleep, resting heart rate, heart rate variability, and respiratory rate were also unchanged (P > 0.05). No differences were observed across the subsequent 6 nights or after the flat run (P > 0.05).

Sleep duration increased on the night after muscle-damaging exercise in hot conditions, with greater REM and SWS reflecting longer sleep rather than altered architecture.

Individuals from various populations, including athletes, military, occupational workers, and the general public may participate in exercise under heat stress, requiring awareness of potential sleep disturbances following exercise in hot conditions.