Endocrine Abstracts

At high latitudes over-wintering personnel are deprived of natural sunlight in winter: light of sufficient intensity and suitable spectral composition is the main factor which maintains a 24 h period in human circadian rhythms. Numerous reports of sleep problems from Polar regions prompted investigations of the underlying mechanisms. At Halley (British Antarctic Survey Base, 750S) the sun does not rise for 3 months in winter and a delay of the melatonin rhythm is observed. This implies that sleep is attempted at a sub-optimal phase. A few people desynchronise from the 24 h day (free-run) and show their intrinsic circadian period, usually >24 h. Sleep is governed by both homeostatic and circadian mechanisms: the circadian delay undoubtedly underlies the delayed sleep. Decrements in sleep efficiency, latency, duration, quality are also seen in winter. A skeleton photoperiod (2×1 h, bright white light) restores summer timing. A single 1 h pulse of light in the morning may be sufficient. Extra ambient light (Philips Bright Light Devices), increased daily maximum/average light exposure from 570/30 lux to approximately 2000/70 lux throughout the day and was associated with benefits for sleep/phase: blue enriched light was more effective than standard white light. Base personnel adapt the circadian system to night work, within a week, in contrast to temperate zones where complete adaptation rarely occurs. A similar situation occurs on high latitude North Sea oil installations, especially when working 1800–0600h. Lack of Declaration of interesting light exposure (and ‘social obligations') is the probable explanation. Many have problems returning to day work, showing circadian desynchrony. Timed light treatment helps to restore normal phase/sleep. Post-prandial response to meals is compromised during periods of desynchrony with evidence of insulin resistance, and elevated triglycerides, risk factors for heart disease. These observations suggest that sub-optimal light conditions are deleterious to health.

Declaration of interest: Conflict of Interest: I am director and major shareholder of Stockgrand Ltd, a company based at the University of Surrey, which performed some of the hormone measurements related to this abstract.

Funding: Declaration of Funding: This work was supported by the British Antarctic Survey, the Medical Research Council, Philips Bright Light Devices, The Antarctic Funding Initiative, The Institute of Petroleum, The Energy Institute, Stockgrand Ltd.