Light at night pollution of the internal clock, a public health issue.

Jun 8, 2018Bulletin de l'Academie nationale de medecine

How nighttime light disrupts the body’s internal clock and affects public health

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Circadian Biology on OpenScience ↗PubMed ↗

Abstract

75% of the total workforce in industrialized countries, including France, is involved in atypical hours that can disrupt the circadian system.

Light exposure, especially at night, can block melatonin secretion, which is critical for regulating the sleep-wake cycle.
Circadian desynchronization may occur due to light exposure timing, which can either advance or delay the internal clock.
Sustained night work is associated with an increased risk of breast cancer, particularly observed in nurses with 3 to 20 years of night shifts.
Health issues linked to light exposure at night include sleep deprivation and suppression of melatonin levels.
Shift work has been classified as probably carcinogenic to humans by the International Agency for Cancer Research due to its impact on circadian rhythms.
Proposed countermeasures to mitigate the effects of night work include melatonin supplementation and exposure to bright light.

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Light is the major synchronizer of the internal clock located in the suprachiasmatic nuclei of the anterior hypothalamus. Retinal ganglion cells contain melanopsin, a photoreceptor with a peak sensitivity to blue wavelength (460-480 nm). Light signal is transmitted from the eye to the clock, then to the pineal gland which produces melatonin, considered as the hand of the clock. Even a weak intensity of light (LEDs, tablets, mobile phones, computers...) is able to block the secretion of melatonin, the hormone of darkness. Light is also able to phase advance or phase delay the circadian system according to the timing of exposure. This Phase Response Curve (PRC) is used to resynchronize the clock in various situations of circadian desynchronization. Exposure to Light at Night (LAN) results in a disruption of the circadian system which is deleterious to health. In industrialized countries, including France, 75 % of the total workforce is estimated to be involved in atypical hours, far from the classical diurnal hours of work. Of interest, shift work and night work involve 15.4 % of the French workforce. A number of epidemiologic studies, peiformed mainly on nurses, showed an association between sustained night work (3 to 20 years) and an increased risk of breast cancer Health problems faced by flight attendants have also been reported, though other causes like exposure to radiations cannot be ruled out. Other deleterious effects are reported in this paper. The potential mechanisms of the deleterious effects of LAN on health are suppression of melatonin andsleep deprivation. The International Agencyfor Cancer Research (IARC) classified shift work that involves circadian disruption as ( probably carcinogenic to humans". Countermeasures (e.g melatonin, bright light, use of psychotropic drugs) have been proposed as a means to improve adaptation to shift work and night work and to fight " clock pollution " and circadian desynchronization by LAN.

Light at night pollution of the internal clock, a public health issue.

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