. 2017 Jan; 7(1): e1017.

Published online 2017 Jan 31. doi: 10.1038/tp.2016.262

Temporal organization of physiology is critical for human health. In the past, humans experienced predictable periods of daily light and dark driven by the solar day, which allowed for entrainment of intrinsic circadian rhythms to the environmental light–dark cycles. Since the adoption of electric light, however, pervasive exposure to nighttime lighting has blurred the boundaries of day and night, making it more difficult to synchronize biological processes. Many systems are under circadian control, including sleep–wake behavior, hormone secretion, cellular function and gene expression. Circadian disruption by nighttime light perturbs those processes and is associated with increasing incidence of certain cancers, metabolic dysfunction and mood disorders. This review focuses on the role of artificial light at night in mood regulation, including mechanisms through which aberrant light exposure affects the brain. Converging evidence suggests that circadian disruption alters the function of brain regions involved in emotion and mood regulation. This occurs through direct neural input from the clock or indirect effects, including altered neuroplasticity, neurotransmission and clock gene expression. Recently, the aberrant light exposure has been recognized for its health effects. This review summarizes the evidence linking aberrant light exposure to mood.



Sleep disturbance is an important factor contributing to the onset and maintenance of mood disorders. 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., , 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. 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. 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., 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. In another cohort of almost 9000 individuals, symptoms of insomnia and snoring were associated with areas of high light pollution. 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. 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., The data seem consistent with light at night deranging circadian clocks, sleep being one of the ‘hands’ of the endogenous biological clocks.

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