Living in Biological Darkness II: Impact of Winter Habitual Daytime Light on Night-Time Sleep.

Timing and architecture of sleep are co-driven by circadian rhythms modulated by their major Zeitgeber light and darkness. In a natural environment, one is exposed to 3.000 lx (cloudy winter sky) to 100.000 lx (bright sunny sky). The aim of the study was to assess (1) habitual daytime light exposure in urban winter and (2) impact of daytime urban light on objective night-time sleep. Eleven healthy participants (mean age ± SD: 25.4 ± 2.8 years; 6 male) wore eyeglass frames continuously recording daytime illuminance levels vertically to the eye by mounted sensors (range: 1-40.000 lx) during four consecutive days in winter 2008 in Berlin, Germany. In-lab polysomnography was performed over two nights in nine participants. Median light exposure over 4 days was the following: full day 7:00-19:00 h: 23 lx (12-37 lx); morning 7:00-11:00 h: 81 lx (19-201 lx); midday 11:00-15:00 h: 68 lx (19-164 lx); afternoon 15:00-19:00 h: 22 lx (6-58 lx), resulting in only 36 min > 500 lx per day. Timing of daytime light intensity was significantly associated with subsequent sleep: lower midday illuminance with shorter REM latency (Rho = 0.817; p = 0.049) and earlier REM polarity (less prevalence of REM at end-of-sleep; Rho = 0.817; p = 0.049). Humans, living in an urban environment, appear to be exposed to extremely low light levels, which we named as 'Living in Biological Darkness'. Most fascinating, physiology seems to adapt and responds to variation in light intensity on such low levels. Interestingly, the observed changes in sleep architecture with low light levels are reminiscent of those suspected to constitute biological markers of depression some 40-50 years ago.