Different environmental conditions, particularly daylength and intensity of natural light, may influence the ability of shiftworkers to adapt to the abrupt phase-shifts of 24 h time cues imposed by the nature of their work. We have investigated this problem in terms of the circadian rhythm of the pineal hormone melatonin in nightshift workers on the British Antarctic Survey Base at Halley (75° South). Melatonin production was assessed by measurement of its major urinary metabolite 6-sulphatoxymelatonin (aMT6s) by radio-immunoassay in sequential urine samples collected for 48 h at weekly intervals. The acrophase of the melatonin rhythm was significantly delayed from 5.22 h.min to 14.54 h.min (summer) and 8.73 h.min to 13.23 h.min (winter) during a week of night-shift work. Readaptation of the rhythm following night-shift work was markedly slower during the Antarctic winter taking 3 weeks compared to summer where the baseline phase position was re-established after 1 week. Morning and evening treatment (08.00–09.00 h, 16.00–17.00 h) with bright (> 2500 lux) full spectrum white light did not significantly modify this phenomenon in summer, but a trend to faster adaptation with light treatment was seen in winter. These observations are likely to be of importance to shift-workers in temperate zones. Further investigations of phase-shifting techniques, such as appropriately timed bright light and administration of melatonin itself, are indicated, particularly in relation to performance at work.