Figure 3

Correlations with latitude for sleep phenotype measured at different circadian times. The histogram bars correspond to the R-square values obtained when the hour-by-hour average sleep bout duration was regressed over population latitude. The fact that none of the 24 regressions was significant after correction for multiple testing but nighttime regressions showing elevated R-square is consistent with the presence of a global daytime vs. nighttime sleep pattern rather than sharp differences occurring over short time periods. The white bar underneath the graph represents the photophase, the black bar the scotophase.