High-intensity exercise in the evening does not disrupt sleep in endurance runners

Purpose To investigate the effect of early evening exercise training at different intensities on nocturnal sleep and cardiac autonomic activity in endurance-trained runners. Methods Eight runners completed three experimental trials in a randomised, counterbalanced order. In the early evening (end of exercise 3.5 h before bedtime), participants performed either: (i) a 1 h high-intensity interval running session (HIGH, 6 x 5 min at 90% VO2peak interspersed with 5 min recovery); (ii) a 1 h low-intensity running session (LOW, 60 min at 45% VO2peak) or (iii) no exercise (CON). Subsequent nocturnal sleep was assessed using polysomnography, wristwatch actigraphy, and subjective sleep quality. A two-lead electrocardiogram recorded nocturnal cardiac autonomic activity. Results Total sleep time increased after HIGH (477.4 +/- 17.7 min, p = 0.022) and LOW (479.6 +/- 15.6 min, p = 0.006) compared with CON (462.9 +/- 19.0 min). Time awake was lower after HIGH (31.8 +/- 18.5 min, p = 0.047) and LOW (30.4 +/- 15.7 min, p = 0.008) compared with CON (46.6 +/- 20.0 min). There were no differences between conditions for actigraphy and subjective sleep quality (p > 0.05). Nocturnal heart rate variability was not different between conditions, but average nocturnal heart rate increased after HIGH (50 +/- 5 beats min(-1)) compared with LOW (47 +/- 5 beats min(-1), p = 0.02) and CON (47 +/- 5 beats min(-1), p = 0.028). Conclusion When performed in the early evening, high-intensity exercise does not disrupt and may even improve subsequent nocturnal sleep in endurance-trained runners, despite increased cardiac autonomic activity. Additionally, low-intensity exercise induced positive changes in sleep behaviour that are comparable to those obtained following high-intensity exercise.