Association between intrinsic circadian period during forced desynchrony (T-Cycle= 28.0) and the phase angle of entrainment on A) day 1 of entry to the laboratory, B) day 7, the first cycle of the constant routine (CRa), and C) day 8, the second cycle of the constant routine (CRb) for subjects in study 2. Symbols represent individual subjects for whom melatonin onset and intrinsic period data were available. The dashed line represents a linear fit of the data. The phase angle of entrainment, as determined by the relationship between melatonin onset and habitual sleep time, is positively and robustly related to intrinsic circadian period (τm) with slopes ranging from 2.42 to 5.25.