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

Abstract

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.

Keywords: REM latency; SWS; biological darkness; depression marker; habitual daytime lighting; sleep.

FIGURE 1

Glasses with mounted Luxblick sensors.

FIGURE 2

Light exposure during habitual daytime activities. Daytime illuminance during 4 days per participant (n = 11). (A) Group averages of 90th percentiles (dark yellow), means (dashed black), medians (bold black) and 10^th percentiles (dark green) of individual illuminances per hour. Smooth line represents dots of hourly means. Sunrise and sunset were derived from meteorological data (www.sunrise‐and‐sunset.com) and averaged for study duration (Berlin, Germany; 14th November – 8th December 2008; sunrise 7:27–8:04; sunset 15:51–16:13). (B) Data assigned to categories: < 20, < 80, < 200 and < 500 lx; expressed as mean percentage of time spent in these categories per hour; standard errors indicated by error bars.

FIGURE 3

Two extreme examples: ‘regular’ versus ‘depression‐like’ hypnogram and daytime illuminance. Upper panels: hypnograms; W = wake, R = REM sleep, N1, N2, N3 = NREM stages N1, N2, N3; N3 is synonymous with SWS; REM latency = elapsed time between sleep onset and the first epoch of REM sleep. Middle panels: prior day median illuminance (lx) per hour at eye level. Bottom panels: summed polarity vectors for REMS (dark blue) and SWS (dark red); these are obtained by considering every 30‐s epoch within the sleep period time as being a vector with standard length 1 and angle expressed by clock time (24:00 h format) and then adding up the vectors of similar respective sleep stages (REMS, light blue; SWS, light red). The angle of the summed vector thus represents a weighted sum, ‘polarity’, having a specific clock time. Its length expresses the strength of the polarity, with low values indicating low presence and/or considerable temporal dispersion of the respective sleep stage. A similar approach has been used to depict the summed polarity vector (dark orange) of the varying daytime illuminance levels (light orange, with lengths corresponding to the respective illuminances in lx, instead of standard length 1). For representation purposes, the strength of the sleep summed vectors in the figure has been divided by 200 and that of illuminance by 10,000. ——— = start of the experimental day; −−−− = sleep onset; −·−·−· = end of sleep. Note the low daytime illuminance associated with the depression‐like sleep pattern, indicated by problem to initiate sleep, early morning awakening, shortened REMS latency, reduced REMS polarity and shift of SWS to later sleep times (nearly overlapping angles and low strengths of the summed polarity vectors for REM and SWS).