Phase-shifting response to light in older adults

Abstract

Age-related changes in circadian rhythms may contribute to the sleep disruption observed in older adults. A reduction in responsiveness to photic stimuli in the circadian timing system has been hypothesized as a possible reason for the advanced circadian phase in older adults. This project compared phase-shifting responses to 2 h of broad-spectrum white light at moderate and high intensities in younger and older adults. Subjects included 29 healthy young (25.1 ± 4.1 years; male to female ratio: 8: 21) and 16 healthy older (66.5 ± 6.0 years; male to female ratio: 5: 11) subjects, who participated in two 4-night and 3-day laboratory stays, separated by at least 3 weeks. Subjects were randomly assigned to one of three different time-points, 8 h before (-8), 3 h before (-3) or 3 h after (+3) the core body temperature minimum (CBTmin) measured on the baseline night. For each condition, subjects were exposed in a randomized order to 2 h light pulses of two intensities (2000 lux and 8000 lux) during the two different laboratory stays. Phase shifts were analysed according to the time of melatonin midpoint on the nights before and after light exposure. Older subjects in this study showed an earlier baseline phase and lower amplitude of melatonin rhythm compared to younger subjects, but there was no evidence of age-related changes in the magnitude or direction of phase shifts of melatonin midpoint in response to 2 h of light at either 2000 lux or 8000 lux. These results indicate that the acute phase-shifting response to moderate- or high-intensity broad spectrum light is not significantly affected by age.

Figure 1. Consort diagram showing the flow of participants

Figure 2. Schematic of the experimental protocol

Subjects were admitted for 4 nights and 3 days, were maintained under dim light during daytime hours and were allowed 8 h of sleep in darkness at their habitual time (dark bars). Blood samples were taken throughout the baseline and post-treatment nights to assess light-induced changes in the circadian melatonin rhythm. On night 3, subjects were exposed to light at one of three time-points (−8 h, −3 h, 3 h) relative to the core body temperature minimum (CBTmin) measured on the baseline night. They were exposed to a 2 h light pulse (including a 15 min ramp up and ramp down) of 2000 lux on one laboratory stay and 8000 lux on a different laboratory stay, in a randomized order, separated by at least 3 weeks.

Figure 3. Timing of light exposure, sleeping and waking when the midpoint of 2 h light exposure (including 15 min ramp-up and ramp-down periods) was targeted at 8 h or 3 h before or 3 h after each subject’s baseline core body temperature minimum (CBTmin)

In this example the CBTmin is at 05.00 h, habitual bedtime is at 23.00 h and habitual waking time is at 07.00 h. With light scheduled 8 h before CBTmin, lights out was scheduled at each subject’s usual bedtime; with light exposure scheduled during the usual sleep period (e.g. 3 h before CBTmin), lights out was scheduled 1 h earlier than the subject’s habitual bedtime, and lights on was scheduled l h later than each subject’s habitual waking time; with light scheduled 3 h after CBTmin, lights out and lights on were scheduled at the subject’s habitual times.

Figure 4. Relative intensity at various wavelengths for the broad-spectrum fluorescent light

Figure 5. Phase response curves for 2 h light pulses at 2000 lux and 8000 lux in younger and older subjects

The magnitude and direction of phase shifts are plotted against the timing of the centre of the light exposure relative to the baseline melatonin midpoint following exposure to light of 2000 lux, A, and 8000 lux, B, at three different times relative to the core body temperature minimum (CBTmin). Positive values indicate a phase advance; negative values indicate a phase delay. The curved lines illustrate non-linear curve fit. C, the fitted curve from the combined data for exposure to light at 2000 lux and 8000 lux.

Figure 6. Magnitude of phase shifts within 2 h bins

Phase shifts were binned into 2 h intervals based on time of exposure to light at 2000 lux and 8000 lux relative to the baseline melatonin midpoint in younger and older subjects.