Differences in Sleep, Light, and Circadian Phase in Offshore 18:00–06:00 h and 19:00–07:00 h Shift Workers

May 20, 2008Chronobiology international

Differences in sleep, light exposure, and body clock timing in offshore workers on 6 PM–6 AM versus 7 PM–7 AM shifts

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Abstract

Subjects working the 19:00-07:00 h shift had a significantly later 6-sulphatoxymelatonin acrophase of 14.6±0.55 h compared to those on the 18:00-06:00 h shift, whose acrophase was 11.7±0.77 h.

Two subjects did not adapt to the 19:00-07:00 h night shift, showing acrophases of 4.3±0.22 h and 5.3±0.29 h.
Mean sleep duration was shorter for the 19:00-07:00 h group at 5.71±0.31 h compared to 6.64±0.33 h for the 18:00-06:00 h group.
A trend towards higher morning light exposure was observed in the 19:00-07:00 h group (p=0.07).
Circadian phase was delayed by an average of 3 hours for those on the 19:00-07:00 h schedule.
The earlier shift start and end time may favor daytime sleep in offshore summer conditions.

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Complaints concerning sleep are high among those who work night shifts; this is in part due to the disturbed relationship between circadian phase and the timing of the sleep-wake cycle. Shift schedule, light exposure, and age are all known to affect adaptation to the night shift. This study investigated circadian phase, sleep, and light exposure in subjects working 18:00-06:00 h and 19:00-07:00 h schedules during summer (May-August). Ten men, aged 46+/-10 yrs (mean+/-SD), worked the 19:00-07:00 h shift schedule for two or three weeks offshore (58 degrees N). Seven men, mean age 41+/-12 yrs, worked the 18:00-06:00 h shift schedule for two weeks offshore (61 degrees N). Circadian phase was assessed by calculating the peak (acrophase) of the 6-sulphatoxymelatonin rhythm measured by radioimmunoassay of sequential urine samples collected for 72 h at the end of the night shift. Objective sleep and light exposure were assessed by actigraphy and subjective sleep diaries. Subjects working 18:00-06:00 h had a 6-sulphatoxymelatonin acrophase of 11.7+/-0.77 h (mean+/-SEM, decimal hours), whereas it was significantly later, 14.6+/-0.55 h (p=0.01), for adapted subjects working 19:00-07:00 h. Two subjects did not adapt to the 19:00-07:00 h night shift (6-sulphatoxymelatonin acrophases being 4.3+/-0.22 and 5.3+/-0.29 h). Actigraphy analysis of sleep duration showed significant differences (p=0.03), with a mean sleep duration for those working 19:00-07:00 h of 5.71+/-0.31 h compared to those working 18:00-06:00 h whose mean sleep duration was 6.64+/-0.33 h. There was a trend to higher morning light exposure (p=0.07) in the 19:00-07:00 h group. Circadian phase was later (delayed on average by 3 h) and objective sleep was shorter with the 19:00-07:00 h than the 18:00-06:00 h shift schedule. In these offshore conditions in summer, the earlier shift start and end time appears to favor daytime sleep.

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Differences in Sleep, Light, and Circadian Phase in Offshore 18:00–06:00 h and 19:00–07:00 h Shift Workers

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