Adaptation of the 6-sulphatoxymelatonin rhythm in shiftworkers on offshore oil installations during a 2-week 12-h night shift

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The circadian rhythms of most shiftworkers do not adapt to night shift. We have studied oil workers on a rotating system involving 2 weeks day shift (0600–1800 h) and 2 weeks night shift (1800–0600 h) throughout a day and night shift sequence. Urine samples were collected 3-hourly whilst awake, with an over-sleep collection, for the measurement of 6-sulphatoxymelatonin by radioimmunoassay. In three separate groups results showed adaptation by delay of the 6-sulphatoxymelatonin rhythm in the first week of night shift. The rates of phase shift (mean±SEM) were 1.51±0.16 h/day ( n=5), 1.32±0.41 h/day ( n=5) and 1.77±0.31 h/day ( n=17). Specific environmental and social factors together with the shift schedule on oil rigs may facilitate adaptation to a 12 h night shift within a week.