Latitudinal cline in nighttime sleep bout duration. Daytime and nighttime population mean phenotypic values (± s.e.m.) are shown respectively for locomotor activity A) and B), sleep bout duration C) and D), and walking speed E) and F). Only sleep bout duration follows a latitudinal cline at nighttime (linear regression: R-square = 0.80; p-value = 0.038) but not at daytime (linear regression: R-square = 0.31; p = 0.32). The red line represents the regression of population phenotypic means (y-axis) over latitude (x-axis). ME: Maine, RI: Rhode Island, VA: Virginia, FL: Florida, and PC: Panama City.