Figure 1.
Protocol for assessing circadian phase shift responses and melatonin suppression.
(A) Subjects took part in a 6-day laboratory study. Circadian rhythms were assessed using constant routine (CR) procedures before and after an experimental light exposure session. During the CR procedure, subjects were exposed to <5 lux of ambient light. During the light exposure session, subjects were exposed to 6 h of continuous red light (631 nm, 13 log photons cm−2 s−1), intermittent red light and darkness (∼1 min on, 1 min off), or bright polychromatic white light (2,500 lux; 4000K) starting 1 h before habitual bedtime. (B) The narrow-bandwidth red light stimulus was generated using a light-emitting diode and delivered to subjects' eyes using a modified Ganzfeld dome. The spectral emission of the LED stimulus is shown.
Figure 2.
Circadian rhythms assessed before and after exposure to light.
Circadian rhythms of melatonin (MLT), cortisol (Cort), core body temperature (CBT), forehead skin temperature (FST), and heart rate (HR) were measured using constant routine procedures. Black traces show rhythms on the day prior to light exposure, and gray traces show rhythms on the day after light exposure. Subjects were exposed to 6 h of continuous red light (left column), intermittent red light and darkness (center column), or bright white light (right column) starting 1 h before habitual bedtime. Results are Z-scored and averaged across subjects with data binned hourly. Vertical gray bars indicate the usual hours of sleep. Since participants had different self-selected bedtimes prior to the study, and study events were timed according to each person's pre-study sleep-wake schedule, analyses are presented using relative clock time, with relative bedtime defined as midnight. Circles with error bars show the mean ± SEM.
Figure 3.
Circadian phase shift responses were similar for exposure to continuous versus intermittent red light.
(A) Circadian responses are shown for individual subjects exposed to 6 h of continuous red light, intermittent red light and darkness, or bright white light. Circadian phase shifts are shown for melatonin (MLT), cortisol (Cort), core body temperature (CBT), forehead skin temperature (FST), and heart rate (HR). By convention, negative values show phase delay shifts. Horizontal lines show the grand mean for each light exposure condition, with the corresponding values shown at the top of the plot. (B) Most subjects exposed to continuous red light exhibited a small resetting response (left), but one participant showed a phase delay shift comparable in magnitude to bright white light exposure (right). (C) Likewise, in response to intermittent red light and darkness, circadian phase measured before and after light exposure was similar in most subjects (left); however one subject showed a large phase delay shift (right). In B and C, representative subjects are shown, with the circadian rhythm of core body temperature (CBT) shown before and after exposure to light (black and gray traces, respectively). Vertical lines show the timing of the fitted minimum for the CBT rhythm. Phase shift values are shown at the top of each plot, and the corresponding subject code is shown on the bottom right.
Table 1.
Circadian phase shift responses to light (h ± SEM).
Figure 4.
Melatonin levels and pupillary constriction during nocturnal light exposure.
(A) Melatonin profiles are shown for participants exposed to 6 h of continuous red light (left), intermittent red light and darkness (center), or bright white light (right) near the onset of melatonin secretion. Black traces show the melatonin rhythm on the day prior to light exposure, and gray traces show melatonin on the day of the light exposure session. Melatonin concentrations during light exposure were individually adjusted using Z-score values obtained during the first constant routine procedure. Vertical dotted lines indicate the onset and offset of the light exposure session. (B) The area under the curve (AUC) of the melatonin profile during light exposure is shown for each subject, expressed as a percentage of his AUC measured in dim light. Values less than 100% therefore indicate light-induced melatonin suppression, whereas values that exceed 100% indicate that the AUC was higher during the light exposure session relative to the AUC measured in dim light on the previous day. The open circles show responses for subjects crl30 and irl31, who exhibited substantial resetting of circadian rhythms, as shown in Figure 3. (C) The pupillary light reflex is shown during the first 50 min of exposure to continuous red light (left), alternating red light and darkness (center), and bright white light (right). (D) The median pupillary light response is shown for individual subjects during the 50-min fixed gaze period, expressed relative to the dark pupil. Horizontal dotted lines in C and D indicate pupil diameter in darkness, and data in C are binned at intervals of 15.625 s, corresponding to one-quarter of an intermittent lights-on pulse. In A and C, the mean ± SEM is shown. In B and D: crl, continuous red light; irl, intermittent red light; bwl, bright white light.