Author(s): KOLLOKAS, A., DARDAMANIS, C., KRIMIZI, I., MILIOTIS, P., GELADAS, N., KOSKOLOU, M., BOTONIS, P. , Institution: UNIVERSITY OF ATHENS, Country: GREECE, Abstract-ID: 340

INTRODUCTION:
Regular physical activity and adequate sleep are fundamental elements for the maintenance of health. With the global increase in ambient temperature, nighttime exercise in the heat has become common among athletes and physically active individuals. However, the effects of exercise performed in the heat on subsequent nocturnal sleep have been poorly investigated. The aim of the present study was to evaluate the influence of acute nighttime vigorous exercise performed in the heat on subsequent nocturnal sleep characteristics in physically active individuals.
METHODS:
In free-living conditions, nocturnal sleep of eleven physically active individuals (age: 27.1 ± 5.4 years; VO₂max: 41.97 ± 3.6 ml·kg⁻¹·min⁻¹; body mass: 74.8±8.79kg; body fat: 10.4 ± 5.9%) was monitored for five consecutive nights (baseline) and following acute exercise in the heat (EXP) using wrist-worn actigraphy. In EXP condition, each exercise session was scheduled to finish approximately 2 h before participant’s habitual bedtime. EXP protocol required participants to complete a bout of nighttime exercise in the heat (30.9 ± 1.0°C, relative humidity 50 ± 1.7%) at an intensity corresponding to 72 ± 6.4% of VO₂max until volitional fatigue. Nocturnal sleep characteristics included time in bed (h), wake-up time (h), sleep onset latency (min), total time in bed (min), total sleep time (min), number of awakenings, wake after sleep onset (min), and sleep efficiency (%). Sleep data were analyzed using paired-sample t-tests, and the magnitude of differences between baseline and EXP was determined using Cohen’s d. Statistical significance was set at P ≤ 0.05.
RESULTS:
In EXP condition, exercise duration was 41.6 ± 11.8 min, and exercise cessation occurred at 22:38 ± 1:15 h, in close proximity to participant’s baseline bedtime (00:45 ± 1:01 h). Time in bed (baseline: 00:45±1:01 h vs. EXP: 01:08±1:24 h, P=0.11) and wake-up time (baseline: 8:10 ±1:07 h vs. EXP: 7:45±1:19 h, P=0.10) were similar between conditions. Similarly, no differences were observed in total bed time (baseline: 444 ±32 min vs. EXP: 397 ±78 min, P=0.06), total sleep time (baseline: 396 ±32 min vs. EXP: 359 ±76 min, P=0.10), wake after sleep onset (baseline: 29 ±19 min vs. EXP: 31 ±14 min, P=0.81) and sleep efficiency (baseline: 89.8 ±2.31% vs. EXP: 90.1 ±3.4%, P=0.84) between conditions. However, sleep onset latency was longer (baseline: 2.75 ± 2.58 min vs EXP: 7.36 ± 5.14 min, P=0.02, d=0.87) and number of awakenings was less in EXP compared to baseline (baseline: 31.39± 13.63 vs EXP: 11.09± 6.9, P= 0.00, d=1.54)
CONCLUSION:
Acute nighttime vigorous exercise performed in the heat significantly delayed sleep. On the other hand, the reduced number of nocturnal awakenings suggests increased homeostatic sleep drive following exercise, indicating that exercise in the heat impairs sleep initiation but may facilitate sleep maintenance.