Predicting Circadian Response to Abrupt Phase Shift: 6-Sulphatoxymelatonin Rhythms in Rotating Shift Workers Offshore

JOURNAL OF BIOLOGICAL RHYTHMS, Vol. 22 No. 4, August 2007 368-370 DOI: 10.1177/0748730407302843 © 2007 Sage Publications Oil installation workers provide a suitable population in which to investigate the circadian response to abrupt change of work schedule in field conditions. Their working conditions enable long-term continuous sequential collection of urine, for circadian phase assessment using the rhythm of 6-sulphatoxymelatonin (aMT6s), a validated measure of the timing and production of melatonin in the field (Ross et al., 1995). A fixed night shift offshore from 1800 to 0600 h favors complete circadian adaptation within a week as assessed by the aMT6s rhythm (Barnes et al., 1998) and by sleep and mood assessment (Bjorvatn et al., 2006). However, a pilot study of “swing” (or “rollover”) shifts, whereby a week of nights (1800-0600 h) was followed by a week of days (0600-1800 h), suggested that while most of the 11 subjects (men) adapted their aMT6s rhythm to nights, the subsequent response to days was variable, with some subjects showing advances, others delays, and many showing only small circadian changes (Gibbs et al., 2002). Using the same protocol, we have now studied a further 12 men working this identical schedule. We have combined the data from these studies to enable a more detailed analysis of circadian response. In addition, we have assessed the possible suppression of melatonin during both the day and night work periods. Activity and light exposure were measured by actigraphy in the new subjects/study. Nineteen of 23 subjects adapted to the night shift by delay of the aMT6s rhythm (Fig. 1). Of the remainder, 1 was preadapted to night shift (D2 acrophase 10.4 h), and 3 did not adapt to nights (acrophases, 3.8, 5.6, and 2.8 h on D2, 3.4, 4.3, and 4.3 h on D7). There were no significant differences with season of study (autumn compared to spring). Seven of the night adaptors showed little phase change on return to the day shift. Six subjects continued to phase-delay, while 6 phase-advanced in response to day work (Fig. 1). Only those subjects who advanced their aMT6s rhythm achieved a full adaptation back to day shift within a week. Subjects who phase-delayed during day shift adapted faster to night shift than those who phase-advanced during day shift (p < 0.01). They had a later initial (day 2) aMT6s acrophase (6.34 ± 1.12 h) than those who did not change phase during day work (4.71 ± 1.64 h), and subjects who advanced their aMT6s rhythm during day work had the earliest acrophase (2.43 ± 0.65 h) (mean ± SD, p < 0.01). In night adapters who delayed or did not change phase during day shift, lower average aMT6s 24-h production was found during day shift compared to night shift (14,338 ± 7102 and 11,446 ± 5967 ng/24 h, mean ± SD, n = 13, paired t test, p < 0.05). Those who advanced circadian phase during the day shift week showed no overall change in aMT6s production during that week. Average light exposure was greater during day work hours (153 ± 59 lx/min) than night work hours (80 ± 10 lx/min, mean ± SD, n = 12, p < 0.01) (Fig. 2). Moreover, subjects who did not adapt to LETTER