Figure 1. Racial differences in the human endogenous circadian period.

The human endogenous circadian period depends on race, season, and age. (a) Histogram of circadian period (tau) for subjects (n = 60) whose self-reported race was African American (black bars) or not African American (white bars). (b) Circadian period by self-reported race. (c) Circadian period by 2 month bins. In (b) and (c), horizontal lines depict the mean. (d) Circadian period by age. Black dots in (c) and (d) indicate African American subjects.