O022 Response of Sleep and the Circadian System to a Long Summer Photoperiod at High Latitude

The light-dark cycle is the primary time cue for the synchronization of the internal human circadian system with the external world. Our previous research has shown the capacity of the circadian system to adapt to seasonal variations in the light-dark cycle at latitude ~40oN in the Rocky Mountains following curtailment of artificial light exposure. In the current study, we examined the circadian response to a longer photoperiod in summer by measuring melatonin timing before and after a week of camping outdoors in Sweden. Actigraphy-derived (Actiwatch Spectrum) assessments of sleep, activity and light exposure were carried out for 2 weeks. Week 1 was under modern urban living conditions with exposure to natural and artificial light in Stockholm, Sweden (~59oN; 18 hr 37 min: 5 hr 23 min light-dark cycle). Week 2 was spent rough camping in nature with exposure to only natural light at the Swedish High Coast (~63oN; 20 hr 40 min: 3 hr 20 min). Dim-light melatonin onset (DLMO) assessments were conducted in laboratory over two 24 hr periods immediately before and after camping to assess melatonin levels in hourly saliva samples. On average, participants’ (n = 10) melatonin midpoint and offset advanced ~0.7 and ~1.3 hr (p < 0.01), respectively, with no change in onset (p = 0.45). As sleep duration, and timing were similar between environments (p > 10), melatonin offset occurred closer to sleep offset after camping. Consistent with prior findings, the end of biological night is timed closer to waketime in the natural environment.