Table 2: Circadian period (τ), baseline circadian phase, sleep schedule, phase angle of entrainment, and phase shifts to 9-h delay of zeitgebers. Mean ± SD.

From: Circadian rhythms of European and African-Americans after a large delay of sleep as in jet lag and night work

 African AmericanEuropean American
N2322
Free-Running Period (τ) (h)24.06 ± 0.1924.31 ± 0.24*
Baseline Circadian Phasea (h:min)21:37 ± 2:0522:35 ± 1:53
Bedtimeb (h:min)23:52 ± 1:080:27 ± 1:11
Waketimeb (h:min)7:528:27
Phase Angle of Entrainmentc (h)−2.3 ± 1.5−1.9 ± 1.5
Phase Delayd (h)−2.4 ± 1.3−3.6 ± 1.0*
  1. *Significant difference between African-Americans and European-Americans (p < 0.001 by 2-tailed t-test).
  2. aThe dim light melatonin onset (DLMO) from a phase assessment after the four 8-h baseline sleep episodes in the lab.
  3. bScheduled baseline bedtime (dark onset). Scheduled wake time (light onset) was always 8 h later.
  4. cThe phase angle of entrainment of the circadian clock to the 24-h zeitgebers during baseline, calculated as the interval from the baseline DLMO to baseline bedtime (dark onset). Negative numbers indicate that the DLMO occurred before bedtime.
  5. dPhase shift of the circadian clock (assessed by the DLMO) after the 9 h delay of zeitgebers.