The duration of light exposure in the morning and early-afternoon affects adaptation to night work

Introduction : The aim of the study was to examine how the tim-ing of daytime sleep in the dark, and thus the timing of daytime light exposure, affects circadian adaptation to a week of simulated night shifts. It was hypothesised that night work would delay the circadian system – and the size of the delay would increase as the duration of exposure to morning and early- afternoon light (MAL) decreased. Methods : So far, 43 adults (21F, 22M) have been randomly assigned to one of four conditions in a laboratory- based simulated shiftwork protocol. Each condition included 7 consecutive 8- h night shifts (23:00–07:00 hr). The only difference between conditions was in the timing of the 7- h sleep opportunities in breaks between shifts – morning (08:30–15:30 hr, shortest MAL), split#1 (08:30–13:30 hr & 19:30–21:30 hr, short MAL), split#2 (08:30–10:30 hr & 16:30–21:30 hr, long MAL), and afternoon/evening (14:30–21:30 hr, longest MAL). Circadian phase was assessed using salivary dim light mela-tonin onset (DLMO) on the nights immediately before and after the week of night work. Light intensity was 75 lux during night shifts, < 0.03 lux during sleep, < 10 lux during DLMO assessments, and 350 lux at other times. Results : The DLMO data were analysed using a mixed- design ANOVA with one within- subjects factor (time: pre/post) and one between- subjects factor (condition). There was a significant interaction (F = 10.6; df = 3,39; p < .0001) – the type and size of the phase shift dif-fered between the conditions, i.e., morning (delay = 5.06 ± 2.11 hr), split#1 (delay = 2.58 ± 2.46 hr), split#2 (delay = 1.30 ± 2.62 hr), and afternoon/evening (advance = 0.71 ± 2.84 hr). Discussion : These data indicate that the timing of daytime sleep, and thus the amount of exposure to morning and early- afternoon light (MAL), substantially affects the degree of circadian adaptation to a week of night work. In situations where a shiftworker wishes to maximise adaptation to night work, the most sleep should be taken in the morning. To minimise adaptation, sleep should occur in the later afternoon and evening.