Today over 80% of the human population and 99% of those living in the United States or Europe experience light pollution. As technology boomed, humans encountered even more sources of light at night, including television and computer screens, smartphones and tablet computers ( Table 1). Night shift work was introduced soon after.
For the first time, humans could effectively artificially extend the day. At the end of the Nineteenth century, with the invention of the electric light bulb, exposure to artificial light at night grew rapidly. 9 A candle placed a meter away casted only 1.0 lux of light.
MANUAL THEBRAIN 9 FULL
A full moon on a clear night illuminated the environment 0.1–0.3 lux, 8 or up to 1.0 lux in the tropics. Circadian disruption is associated with a number of negative health effects, including effects on mood, metabolism, cancer risk and the immune system.īefore the advent of electric lights about a century ago, humans were exposed to minimal light at night. 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. Health consequences of aberrant light exposure. We focus below on human exposure to light at night and its consequences for circadian rhythms and mood. 2 Exposure to artificial light at night is linked to risk of breast cancer, metabolic disorders and psychiatric and behavioral disorders 3, 4, 5, 6, 7 ( Figure 1). 1 For humans, health consequences of circadian disruption by light at night are becoming increasingly apparent. A growing body of ecological research points to nighttime lighting near urban areas as a major disruption to wildlife migration, foraging activities, reproduction and immune function. Circadian disruption has become prevalent among wildlife and humans.
Over the past century, the boundaries of day and night have been obscured by the widespread adoption of electric light at night. For humans, temporal organization of physiology is equally important for health and wellness. For nonhuman animals, restricting activities to the appropriate temporal niche is crucial to fitness and survival. Predictable daily cycles of light and dark allow for the synchronization of behavioral and biological processes to the external environment. Organisms on Earth are adapted to the 24-h solar day.
This review summarizes the evidence linking aberrant light exposure to mood. Recently, the aberrant light exposure has been recognized for its health effects. This occurs through direct neural input from the clock or indirect effects, including altered neuroplasticity, neurotransmission and clock gene expression. Converging evidence suggests that circadian disruption alters the function of brain regions involved in emotion and mood regulation. This review focuses on the role of artificial light at night in mood regulation, including mechanisms through which aberrant light exposure affects the brain. Circadian disruption by nighttime light perturbs those processes and is associated with increasing incidence of certain cancers, metabolic dysfunction and mood disorders. Many systems are under circadian control, including sleep–wake behavior, hormone secretion, cellular function and gene expression. 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. 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. Temporal organization of physiology is critical for human health.