3.1. Subjects and Procedures
To examine sleep duration and the circadian effect on sleep duration in athletes performing under extreme circumstances, a team of Roparun athletes was studied. The Roparun is a 500 kilometres relay race from Paris or Hamburg to Rotterdam. The Roparun team participating in the study consisted of ten athletes (9 men, 1 woman): two groups of four runners and one cyclist. The two groups alternated in about five hours running and five hours resting. During running, batons were passed on between the same four runners who ran two kilometres each for 60 consecutive kilometres, accompanied by one supporting cyclist. After 60 kilometres the other group of four runners and one cyclist took over. While one group performed, the other group was able to rest, eat, and sleep. So athletes could not sleep for long continuous periods and they had to perform shortly before going to sleep.
One week before the start of the Roparun, all athletes received basic information on optimal sleep hygiene, including the advice to extend sleep time in the week before the competition by trying to go to bed one hour earlier than usual, to prevent excitement in the time between finishing the individual run and sleep onset, and to sleep at a place as quiet and comfortable possible.
Informed consent was obtained from all study participants. The study conforms to the ethical guidelines of the 1975 Declaration of Helsinki.
3.2. Outcome Measures
Sleep duration was assessed using actigraphy (Vivago Ultra Watch, Zoetermeer, The Netherlands). Actigraphy is considered a reasonable valid and reliable method for objectively assessing sleep in comparison to polysomnography (10). The athletes wore the actigraphs two days before the start of the run to investigate possible effects of sleep duration before the race on sleep duration during the race. They kept wearing the actigraphs until the last run. See Figure 1 for an example of the actigraph output of one of the runners.
Figure 1.
Example Activity Curve for One Athlete
The black spikes indicate wrist movements. When there is no wrist movement the runner is considered to be asleep, indicated by a horizontal bar. The running periods are indicated in the graph by dotted lines.
If the run was finished between 22:00 - 3:00 hours, we considered this as “favourable”, in view of what would be the best time to fall asleep according to the circadian rhythm. Bed times outside this time window were considered “unfavourable”. The window of optimal bed times was determined on the basis of the circadian process of sleep, which states that it is easiest to fall asleep at times when melatonin secretion is high and increasing (5). Exact times vary between individuals, but as all participants held daytime jobs, we chose this time window as, on average, persons who have 23:00 hour as a regular bedtime, have their highest melatonin secretion between 22:00 hour and 3:00 hour (11).
3.3. Analysis
Data were analyzed using a linear mixed model (computer program IBM SPSS Statistics version 20, Armonk NY, United States of America), with a compound symmetry covariance structure to account for the dependency of repeated observations (12). The mixed model enables the analysis of data that are a combination of independent observations (two groups of five subjects) and dependent observations (repeated measurements), simultaneously testing both within-subject and between-subject effects of falling asleep within or outside the window of favourable bed times.
The dependent variable was sleep duration in each of the four sleep periods during the race. Independent variables were bed time (inside or outside the favourable time window) and control variables sleep period (first, second, third, fourth), relay group (1 or 2), and the amount of sleep prior to the race (total hours of sleep during the two nights before the race). Regression coefficients indicate the change in the dependent variable (sleep duration during the race) when the independent variable increases with one unit.
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