Fig. 4.

Relative frequency distributions of acrophases of the activity rhythm (measured with waist-worn accelerometers) in the United States during long days of late spring, summer, and early autumn (14.3 hours of light per day, N = 278) and short days of late autumn, winter, and early spring (10.7 hours of light per day, N = 130). The spread of the distributions, and the modal and mean acrophases, are similar despite the difference in day length.