Endocrine Abstracts

Background

Individuals who do day/night shifts or who are exposed to 24 hours of daylight have a greater CVS risk. This may relate to effects on the diurnal rhythm of sleep, appetite or cortisol.

Aims

To investigate the variations in the diurnal cortisol secretion, appetite and sleep in young healthy civilians from the UK during a five week expedition to Svalbard in the Arctic with 24 hours of daylight.

Method

Salivary cortisol was measured at 0900, 1400, 1800, 2300 on one day 2 weeks before the expedition (visit 1), 2 weeks during the expedition (visit 2) and 2 weeks after returning to the UK (visit 3). Stability of salivary cortisol was measured by taking two 0900 samples on visit 1. One sample was analysed immediately and one stored in the Arctic for analysis after the expedition. Appetite and sleep scores were measured at hourly intervals from 0800 to 2300 on the same days as salivary cortisols using a standardised questionnaire. There was similar calorific intake (4000 kcal) across the group as well as activity. All slept in tents and ambient temperatures ranged from 5°C to – 20°C (including wind-chill factor)

Results

24 volunteers (14F: 10M) completed the study. We found no significant difference in the two 0900 salivary samples taken on visit 1(N=20, P=0.26). During the expedition, cortisol secretion was elevated (mean 10.99 nmol/l, P=0.005) compared with pre (5.61 nmol/l) and post expedition (5.85 nmol/l). Diurnal rhythm persisted during the expedition but with an increased cortisol surge at 0900 compared with visits 1 or 3. There was no significant difference in sleepiness scores or in satiety parameters. Desire to eat was increased during the expedition (P=0.03).

Conclusion

Salivary cortisol is stable over a number of weeks following storage at a variety of temperatures. Exposure to 24 hour daylight and the Arctic climate does not change the diurnal rhythm of cortisol secretion but exaggerates the 0900 surge resulting in a higher level of cortisol over 24 hours. Despite twenty four hour daylight, alertness proved unchanged. Appetite was increased although satiety was unaffected. These findings suggest that an increase in appetite and cortisol levels may play a role in the increased risk of CVS disease in subjects living in 24 hour daylight.