Changes in human heart rate during sleep following daily physical exercise

Four fit, healthy young men (aged 20) volunteered for the experiment. After a 5 day control period, they marched for 6 consecutive days (from 09:00 h to 17:00 h) for 34 km/day at a speed of 6 km/h, with an energy expenditure of 35% of individual VO2 max. A recovery period of 5 consecutive days began immediately after the exercise period. Sleep records and electrocardiograms were taken every night during the three periods from 22:00 h to 06:00 h. During the exercise period the night time heart rates increased by about 10%, compared to the previous control condition, and returned to normal during the recovery period. The relation between heart rate changes and sleep stages remained identical throughout the three experimental periods. Three subjects showed an increase in heart rate during paradoxical sleep, compared to the preceding slow wave sleep, while one subject experienced the reverse. The tonic increases in heart rate are discussed in relation to changes in body temperature, sleep patterns, blood composition and hormonal status induced by the physical exercise performed.