Sex hormones, including testosterone, are hypothesized to have an influence on the human circadian system. We sampled male students in the period after adolescence. We used the Composite Scale of Morningness (CSM) to assess chronotype and saliva testosterone sampling in 106 University students (23.87±3.56 years; range 19-37) between 26.4.2011 and 6.5.2011, always between 0800h and 0900h. There was a significant negative relationship between CSM scores and saliva testosterone (r(s)=-0.220, p=0.023, two-tailed test) but not between testosterone and average sleep length. Age and testosterone did not correlate with each other nor did age and CSM scores. Our data suggest that chronotype in men might be influenced by testosterone and that high testosterone levels lead to a stronger evening-orientation. Sleep duration was uncorrelated with testosterone, suggesting that timing of sleep - rather than sleep length itself - is influenced by testosterone.
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