Intrinsic near-24-h pacemaker period determines limits of circadian entrainment to a weak synchronizer in humans

Abstract

Endogenous circadian clocks are robust regulators of physiology and behavior. Synchronization or entrainment of biological clocks to environmental time is adaptive and important for physiological homeostasis and for the proper timing of species-specific behaviors. We studied subjects in the laboratory for up to 55 days each to determine the ability to entrain the human clock to a weak circadian synchronizing stimulus [scheduled activity-rest cycle in very dim (approximately 1.5 lux in the angle of gaze) light-dark cycle] at three approximately 24-h periods: 23.5, 24.0, and 24.6 h. These studies allowed us to test two competing hypotheses as to whether the period of the human circadian pacemaker is near to or much longer than 24 h. We report here that imposition of a sleep-wake schedule with exposure to the equivalent of candle light during wakefulness and darkness during sleep is usually sufficient to maintain circadian entrainment to the 24-h day but not to a 23.5- or 24.6-h day. Our results demonstrate functionally that, in normally entrained sighted adults, the average intrinsic circadian period of the human biological clock is very close to 24 h. Either exposure to very dim light and/or the scheduled sleep-wake cycle itself can entrain this near-24-h intrinsic period of the human circadian pacemaker to the 24-h day.

Figures

Figure 1

Melatonin onset (DLMO25%) times. (a) Subjects scheduled to the 23.5-h day. Data are plotted to a relative clock time with lights out assigned a value of 2400 h on baseline day 1. Black bars represent scheduled sleep. During the imposed 23.5-h segment, lights out and lights on are advanced by 30 min each day. Melatonin onset occurred near to, but advanced relative to, lights out for the subjects during baseline days, whereas during T = 23.5-h days, melatonin onset progressively phase delays relative to lights out. These data demonstrate failure to entrain the circadian pacemaker to the scheduled 23.5-h wakefulness–sleep light–dark cycle. (b) Subjects scheduled to the 24.0-h day. During the imposed 24.0-h segment, melatonin onset appears stable and occurs near to, but advanced relative to, lights out for half of the subjects and delayed for the other half. These data demonstrate that most subjects entrained to the 24.0-h day with a new phase angle, as would be expected to occur in response to the weak environmental synchronizer. A greater dispersion of melatonin onsets can be observed during forced desynchrony, reflecting individual differences in intrinsic circadian period. In two of six subjects, circadian period advanced and in the remaining subjects circadian period delayed during forced desynchrony (Table 1). Subject 18G1 failed to entrain to the 24.0-h day. (c) Subjects scheduled to the 24.6-h day. During the imposed 24.6-h segment, lights out and lights on are delayed by 36 min each day. Melatonin onset occurs near to, but advanced relative to, lights out for half of the subjects and delayed for the other half during baseline days, whereas during the 24.6-h segment, melatonin onset is progressively phase advanced relative to lights out. These data demonstrate a failure to entrain the human circadian pacemaker to the scheduled 24.6-h day. Individual differences in intrinsic circadian period result in a large dispersion of melatonin onsets during the T = 24.6- and T = 28.0-h forced desynchrony protocols.

Figure 2

Association between phase angle of melatonin onset and scheduled sleep time during the T = 24.0-h segment and intrinsic circadian period during forced desynchrony (T = 28.0 h) for entrained subjects. Symbols represent individual subjects for whom phase angle and intrinsic period data were available (four of five entrained subjects). The change in the phase angle of entrainment between melatonin onset and the scheduled 24.0-h day, as determined on CR1 and CR2, is negatively and robustly related to intrinsic circadian period (τm) with a slope of −11.12. Subjects with a circadian period shorter than 24.0 h advanced, and subjects with a period longer than 24.0 h delayed, such that for every 0.1-h change in circadian period, there was a 1.11-h change in the phase angle of entrainment.

Figure 3

Observed circadian periods during forced desynchrony T = 28.0 h and during (a) T = 24.0 h and (b) T = 24.6 h protocols. Symbols represent individual subjects for whom observed and intrinsic period data were available. The solid black line represents the line of equality, the dashed line represents a linear fit of the data, and the dotted lines represent the target period to entrain to. The average intrinsic circadian period was similar for both groups of subjects during the forced desynchrony [average ± SD (a) 24.07 ± 0.19 h vs. (b) 24.06 ± 0.20 h]. The circadian period lengthened closer to 24.0 h in two subjects and shortened closer to 24.0 h in three subjects during T = 24.0 h compared with their intrinsic circadian period (τm), as assessed during T = 28.0 h. The absolute change in period was significantly greater than 0 (one sample t test: P < 0.05). Subjects scheduled to the 24.6-h day showed a significantly longer observed period during T = 24.6 h (24.19 ± 0.23 h) compared with T = 28.0 h (t test: P < 0.01). These data demonstrate coupling between the endogenous circadian pacemaker and the weak environmental time cues, showing that environmental and/or behavioral periodicity can significantly influence the observed period of the pacemaker when the day length is close to the pacemakers intrinsic period regardless of whether the pacemaker entrains to the environmental day length.