Light exposure patterns in healthy older and young adults

Abstract

Aging is associated with an earlier timing of circadian rhythms and a shorter phase angle between wake time and the timing of melatonin secretion or the core body temperature nadir. Light has a phase-dependent effect on the circadian pacemaker, and modifications of habitual light exposure in older people could contribute to a change in the timing of circadian rhythms or in the phase angle of entrainment. In this study, we compare natural light exposure of community-dwelling older and young subjects studied at the same time of year, focusing on the pattern of light exposure across the waking day. We recorded light exposure data for 3 to 8 days from 22 older (aged 66.01 +/- 5.83) and 22 young subjects (aged 23.41 +/- 4.57), living at home on self-selected sleepwake schedules, and matched for time of year. All subjects were from New England (latitude 42.3 degrees N to 43 degrees N). We compared the percentage of the waking day spent by older and young subjects at 4 different light levels (from very dim to very bright). We compared hourly averaged light exposure data in each group according to clock time and with respect to each subject's daily sleepwake times. Although both age groups spent more than half of their waking hours in dim or moderate room light intensity (<100 lux), we found that the older subjects spent a significantly greater percentage of their waking day in the brighter light levels (> or =1000 lux); their hourly averaged light exposure levels were also significantly greater whether we examined the data with respect to absolute clock time, to wake time, or to bed time, and this was true across all seasons. We found that healthy older people were exposed to significantly higher levels of light throughout their waking day than young people. Differences in natural light exposure may contribute to the age-related phase advance of the circadian pacemaker and its later timing relative to the sleepwake cycle. This hypothesis should be explored further in carefully designed prospective studies.

Figure 1

Average ± SD distribution of light threshold levels (<10 lux, 10 to <100 lux, 100 to <1,000 lux, ≥1,000 lux) in older (black bars) and young (white bars) subjects across the waking day.

Figure 2

Light exposure during waking hours with respect to absolute clock time in older and young subjects. Light exposure levels (in lux) are shown on a logarithmic scale. Mean light exposure data for each waking clock hour were averaged per subject, and then averaged across subjects (± SEM) in each age group. Only those clock hours with at least 3 subjects are presented. Older subjects: filled symbols; younger subjects: open symbols.

Figure 3

Light exposure pattern relative to the 8 hours after wake time and relative to the 8 hours before bedtime in older and young subjects. Light exposure levels (in lux) are shown on a logarithmic scale. Mean light exposure data for each hour in the 8 hours following wake time and in the 8 hours preceding bedtime were averaged per subject, and then averaged across subjects (± SEM) in each age group. Older subjects: filled symbols; younger subjects: open symbols.