Figure 11. Group-average 24 h melatonin profiles referenced to external clock-time and internal circadian time.

Group-average 24 h profiles (±s.e.m.) of plasma melatonin levels in constant dim light in 55 healthy individuals (23 men and 32 women) who slept according to their habitual sleep schedules and remained in constant dim (< 1 lx) light when they were awake. A,melatonin data are averaged across subjects with reference to clock-time. B, same data averaged with reference to onset (up-arrow) and offset (down-arrow) of melatonin secretion (B), as described for A and B in Fig. 3. Because individuals differ in schedules to which their circadian rhythms are entrained and in phase angles at which their circadian rhythms are entrained to these schedules, abrupt changes that are characteristic of rhythm waveforms in individuals, such as onset of melatonin secretion (see examples in Fig. 1), are obscured when data are averaged relative to external phase markers (A). These features are preserved when data are averaged relative to internal phase markers (B). Data extracted from Wehr et al. (2001).