Abstract
Differences in daily light exposure profiles have been reported, with younger M-types shown to spend more time in bright light, especially in the morning, compared with E-types. This study aimed to investigate how patterns of daily light exposure in older non-resident M-types and E-types compare. Sleep diaries were kept during actigraphic measurement of activity and light using the Actiwatch-L for 14 days in 12 M-types [eight females, mean ± standard deviation (SD) Horne–Östberg Morning–Eveningness Questionnaire (HÖ MEQ) score 75.2 ± 1.6] and 11 E-types (seven females, HÖ MEQ 41.5 ± 4.8), over 60 years old, living in their own homes. Light data were log-transformed, averaged over each hour, and group × time analysis of covariance (ancova) performed with age as a covariate. M-types had significantly earlier bed and wake time than E-types, but there was no significant difference in sleep duration, sleep efficiency or time spent in bed between groups. Daily exposure to light intensity greater than 1000 lux was compared between the two groups, with no significant difference in the duration of exposure to >1000 lux between M-types and E-types. Twenty-four-hour patterns of light exposure show that M-types were exposed to higher light intensity at 06:00 h than E-types. Conversely, E-types were exposed to higher light intensity between 22:00 and 23:00 h than M-types. These findings show that differences in daily light exposure patterns found previously in younger M-types and E-types are also found in older M-types and E-types, but at an earlier clock-time, confirming the tendency to advance with ageing.