P052 Is Circadian Adaptation to Night Work Affected by the Intensity of Ambient Lighting?

INTRODUCTION: The aim of this study was to examine whether the intensity of ambient lighting affects the rate at which the human circadian system adapts to working at night. Methods. The whole study will include 60 participants (50:50, F:M; aged 28–35yr; good sleep/health) randomised to one of three conditions. In each condition, participants work 14 x 12-hr simulated night shifts (19:00–07:00h) while living 24h/day in an accommodation suite. The only difference between conditions is in the light intensity during night shifts – dim (5–10lx), moderate (50–100lx), normal (300–350lx). Circadian adaptation is being assessed using the hourly rate of 6-sulphatoxymelatonin (aMT6s) production during sleep. RESULTS: The project is in progress, so these results are for the first 6–12 participants in each condition. For each day, circadian adaptation is assessed by expressing the rate of aMT6s excreted during the daytime sleep period as a percentage of the rate of aMT6s excreted during the night-time baseline sleep period (for an adaption score of 0–100%). On average, participants in the dim condition are adapted by 26%,37%,32% after nights 1,7,14; participants in the moderate condition are adapted by 23%,37%,60% after nights 1,7,14; and participants in the normal condition are adapted by 27%,117%,103% after nights 1,7,14. DISCUSSION: These data indicate that the rate, and degree, of circadian adaptation to night work is substantially affected by the intensity of ambient lighting. Therefore, lighting conditions should be incorporated into OHS guidelines related to managing the fatigue risks associated with night work.