Characterizing the modern light environment and its influence on circadian rhythms

Humans have largely supplanted natural light cycles with a variety of
electric light sources and schedules misaligned with day-night cycles.
Circadian disruption has been linked to a number of disease processes, but
the extent of circadian disruption among the population is unknown. In
this study, we measured light exposure and wrist temperature among
residents of an urban area during each of the four seasons, as well as
light illuminance in nearby outdoor locations. Daily light exposure was
significantly lower for individuals, compared to outdoor light sensors,
across all four seasons. There was also little seasonal variation in the
realized photoperiod experienced by individuals, with the only significant
difference occurring between winter and summer. We tested the hypothesis
that differential light exposure impacts circadian phase timing, detected
via the wrist temperature rhythm. To determine the influence of light
exposure on circadian rhythms, we modeled the impact of morning and
nighttime light exposure on the timing of the maximum wrist temperature.
We found that morning and nighttime light exposure had significant but
opposing impacts on maximum wrist temperature timing. Our results
demonstrate that, within the range of exposure seen in everyday life,
nighttime light can delay the onset of the maximum wrist temperature,
while morning light can lead to earlier onset. Our results demonstrate
that humans are minimizing natural seasonal differences in light exposure,
and that circadian shifts and disruptions may be a more regular occurrence
in the general population than is currently recognized.