SEASONAL AND DIURNAL PATTERNS OF HUMAN ILLUMINATION UNDER NATURAL CONDITIONS

Little is known about the natural pattern of seasonal and diurnal illu mination to which normal people are exposed, especially in northern la titudes. In this study, ambient illumination of normal volunteers livi ng at a latitude of 45 degrees 31' N was recorded with ambulatory phot osensors worn for 5 to 6 days in winter and summer. Results from 12 no rmal subjects (6 men, 6 women) aged 18 to 35 years were included in th e analyses. The mean daily duration of time awake was similar in both seasons: 14.6h in the summer and 14.9h in the winter. However, the pha se of the sleep-wake cycle was advanced in the summer compared to the winter, as shown by an earlier average waketime and bedtime in the sum mer. Illumination recorded by the ambulatory monitor between waketime and bedtime was categorized according to four ranges of light intensit ies: very dim (<1 lux), dim (1-100 lux), moderate (100-1000 lux), and bright (> 1000 lux) illumination. There was no seasonal difference for the time spent in illumination lower than 1000 lux, but the duration of daily exposure to bright light averaged 2.6h in the summer compared to only 0.4h in the winter (p = 0.0004). To evaluate the diurnal dist ribution of ambient illumination, time spent awake was divided into fo ur time intervals: morning (waketime to 12:00), afternoon (12:00 to 16 :00), early evening (16:00 to 20:00), and late evening (20:00 to bedti me). Except for late evening, the time spent in bright illumination wa s significantly longer during the summer for all time intervals, but t he relative distribution of bright light exposure throughout the day w as the same in both seasons. The subjects spent more than 50% of their time awake in illumination dimmer than 100 lux, even in the summer. M ore naturalistic studies are needed to determine whether very short ex posure to bright light or longer exposure to light of moderate intensi ty (100-1000 lux) are sufficient to maintain circadian entrainment and euthymia in normal young subjects.