Endocrine Abstracts

Light of sufficient intensity and suitable spectral composition is the main factor which maintains a 24 h period in human circadian rhythms. At the British Antarctic Survey base of Halley (75 °S) the sun does not rise for 3 months in winter and does not set for 3 months in summer. In this isolated environment most subjects remain synchronised to the 24 h day however the melatonin rhythm and sleep are delayed in winter compared to summer. Summer timing can be restored with a skeleton photoperiod of standard bright white light. Decrements in several other sleep parameters (efficiency, latency, duration, quality) are seen in winter. The introduction of extra light (Philips Bright Light Devices), sufficient to increase daily maximum/average light exposure in winter from 570/30 lux to ~2000/70 lux is associated with benefits for sleep and there is some evidence that blue enriched light is more effective than standard white light. The majority of base members adapt to night shift within a week, assessed by the melatonin rhythm, in contrast to night shift workers in temperate zones who rarely adapt completely. Their sleep improves during night work and in winter they have problems returning to day work, evidenced by free-run or circadian desynchrony. Timed light treatment also addresses this problem. Post-prandial response to meals is compromised during periods of night shift 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.

In high latitudes in winter, short photoperiods together with an interior workplace are associated with a lack of bright natural light. Workstations may have no windows and outdoor activity during daylight hours can be difficult if not undesirable in poor weather conditions. Thus data from Polar regions may well have applicability in such situations.