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Timing of light exposure affects mood and brain circuits - PubMed

T A Bedrosian  1 , R J Nelson  2

Review

Timing of light exposure affects mood and brain circuits

T A Bedrosian et al. Transl Psychiatry. .

Abstract

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.

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Figures

Figure 1

Health consequences of aberrant light exposure. Exposure to light at night or other circadian disruption can perturb synchronization of the central pacemaker in the suprachiasmatic nuclei (SCN) with peripheral clocks throughout the brain and body. Circadian disruption is associated with a number of negative health effects, including effects on mood, metabolism, cancer risk and the immune system.

Figure 2

Satellite images of Earth at night. Images acquired by the Suomi National Polar-orbiting Partnership satellite in 2012 demonstrating urban light pollution (

http://earthobservatory.nasa.gov/

).

Figure 3

Transition to blue-shifted LED bulbs. Image of Milan, Italy taken in 2015 from the International Space Station after the transition to LED bulbs in the city center. The amount of blue light, known to most strongly stimulate the circadian system, is much higher compared with the surrounding suburbs (

http://www.iau.org/public/images/detail/iau1510a/

). LED, light-emitting diode.

Figure 4

Mechanisms of depressed mood caused by exposure to light at night. Studies in rodent models have revealed several putative mechanisms through which exposure to artificial light at night disrupts mood. Nighttime light can indirectly affect mood by disrupting sleep, hormone secretion, neuroplasticity, neurotransmission or gene expression. In parallel, nighttime light can directly affect mood through aberrant signals transmitted from ipRGCs in the retina to brain regions involved in emotional regulation. In the panel, green brain regions represent primary ipRGC targets and blue regions represent secondary targets. HPC, hippocampus; ipRGC, intrinsically photosensitive retinal ganglion cells; LC, locus coeruleus; LHb, lateral habenula; MeA, medial amygdala; SCN, suprachiasmatic nucleus of the hypothalamus; VTA, ventral tegmental area.

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