Effects of sleep loss and strenuous physical activity on the rest-activity circadian rhythm: a study on 500 km and 1,000 km dogsled racers.

Objective: Rest-activity circadian rhythm is strongly linked to an organism's entrainment. Sleep loss and prolonged fatigue could affect the circadian system, inducing neurobehavioral deficits. The Finnmarksløpet is Europe's longest dogsled race. In this competition, lasting up to 7 days, participants (mushers) are physically active most of the time, having little and fragmented rest. Therefore, the race provides an opportunity to investigate the effects of prolonged fatigue and sleep loss.

Methods: Ten mushers, participating in the 500 km and 1,000 km categories, underwent continuous actigraph monitoring (5 days) before and after the race. During the competition, heart rate (HR) was recorded by an HR monitor.

Results: There was a reduction in the average activity values during the 24-hr cycle after the race. Although there were signs of a forward phase shift, these were weak and unstable. Nonparametric circadian rhythm analysis (NPCRA) showed reductions in interdaily stability (IS) and relative amplitude (RA). 1,000 km mushers also showed sleep disturbances.

Conclusion: A period of 3 days of little and fragmented sleep (3-4 hr of total rest per day), in which subjects were engaged in a prolonged physical effort, was enough to significantly affect the rest-activity rhythm. A longer period (5 days) in such a condition induced even more accentuated alterations, with a disturbance in nocturnal sleep. Disrupted sleep is common among hospitalized patients and those working long shifts. This study demonstrates changes in the structure of the rest-activity circadian rhythm that can result and may suggest opportunities for intervention.