Figure 2 : Frequency histograms of the endogenous free-running circadian periods (τ) for the European-Americans (N = 22), African-Americans (N = 23), and these subjects combined (N = 45).

From: Circadian rhythms of European and African-Americans after a large delay of sleep as in jet lag and night work

Figure 2

The free-running periods were calculated from the circadian phase assessments which were before and after the 3 days of ultradian light/dark cycles (see Fig. 1). The means ± SDs were 24.31 ± 0.24, 24.06 ± 0.19, and 24.19 ± 0.24 for the European-Americans, African-Americans and both, respectively.