Timing of light exposure affects mood and brain circuits

Sleep disturbance is an important factor contributing to the onset and maintenance of mood disorders.⁴⁸ One of the most obvious potential effects of nighttime lighting is sleep disruption. During dark nights, melatonin concentrations rise to promote sleep onset and regulate circadian sleep phase.^{49, 50, 51} Exposure to light at night strongly suppresses melatonin, which interferes with sleep timing and sleep quality. Individuals who used an eReader (30–50 lux) during the 4 h before bedtime experienced increased latency to fall asleep and decreased morning alertness. The symptoms coincided with a significant suppression of melatonin.³⁸ Similarly, individuals who slept with a night light of ~40 lux had shallower sleep and increased arousals, as well as markedly decreased brain oscillations during sleep.⁵² Brain oscillations during sleep have been linked to cognition; so it is possible that even relatively low levels of nighttime light could have detrimental effects on cognitive function through sleep disruption.^{53, 54} Interestingly, indoor light pollution is not the only source of sleep disruption. Two large epidemiological studies reported that even light pollution outside the home was sufficient to disrupt sleep. In a study of more than 19 000 individuals, those living in areas with more outdoor night lighting self-reported less quality and quantity of sleep, as well as more daytime sleepiness.⁵⁵ In another cohort of almost 9000 individuals, symptoms of insomnia and snoring were associated with areas of high light pollution.⁵⁶ Although mood was not investigated in these studies, sleep and emotional regulation are intimately linked; sleep disruption is one potential mechanism through which light at night is implicated in depressed mood.⁵⁷ However, sleep is likely only one of several contributors because nocturnal mice develop depressive responses after exposure to light at night without experiencing any sleep disruption.^{58, 59} The data seem consistent with light at night deranging circadian clocks, sleep being one of the ‘hands’ of the endogenous biological clocks.