Review articleAssociation between light at night, melatonin secretion, sleep deprivation, and the internal clock: Health impacts and mechanisms of circadian disruption
Section snippets
Introduction: The internal clock and the circadian system
Circadian rhythms are endogenous rhythms with a periodicity of approximately 24 h (24 ± 4 h). They are widespread and regulate most, if not all, of the major physiological systems in mammals. Circadian rhythms are unquestionably the most studied in the literature though other periods exist that range from milliseconds (i.e. ultradian rhythms, for which the period extends from milliseconds to 20 h) to a year (i.e. infradian rhythms, for which the period extends from 28 h to a year) [1]. Circadian
Light control of melatonin secretion
The circadian system in human beings is a complex entity that starts in the eye and that terminates in the pineal gland, which produces melatonin (5-methoxy-N-acetyltryptamine), a neurohormone essential for functioning of the clock. In humans, melatonin is secreted during the dark phase of the light–dark cycle. Daytime melatonin levels are hence comparatively very low. Light is considered to be the most potent circadian synchronizer for humans, although non-photic time cues, such as meal times,
Artificial light at night disrupts the circadian system
In order to accommodate production demands e.g. machines working 24/24 h, cost saving procedures, or specific issues related to security, approximately 20% of the working population in industrialized countries is engaged in shift and/or night work. This type of employment hence exposes a large number of workers to unusual light-dark cycles. Indeed, in our modern industrialized societies, regular 08 h to 17 h working hours for 5 days a week have become relatively rare (25%) compared to so-called
Shift work, light at night and cancer risk
First hypothesized by Stevens (88), the relationship between exposure to light during shift and/or night work and the occurrence of cancer in workers has been the subject of numerous scientific studies [reviews in e.g. [40], [41], [42]].
Mechanistic approach of ALAN effects in cancer
Various mechanisms have been proposed to explain the effects of light at night on cancer, of which three seem to be essential in impacting various levels of the organism's metabolism: inhibition of night time secretion of melatonin by light, sleep deprivation, and chronodisruption. The reported rise in cancer risk may be a consequence of one -or most probably simultaneous effects- of these three major mechanisms.
Epidemiology
Based on the prospective cohort study of the Nurses' Health Studies, a longer duration of rotating night work was associated with a small yet statistically significant absolute increase in the risk of coronary heart disease [136]. In a systematic review and meta-analysis shift work was significantly associated with myocardial infarction (RR 1.23), ischaemic stroke (RR 1.05), coronary events (RR 1.24) but was not associated with increased rates of mortality (whether vascular cause specific or
Circadian misalignment and diabetes
ALAN appears to favor cardio-metabolic risks [141] which are themselves risk factors for cancers.
Epidemiology
The association of exposure to light at night and being overweight has been found statistically significant [156] in a study using data on countries worldwide (satellite images of night time illumination). Another study of the same kind in South Korea provided epidemiological evidence that ALAN is moderately but significantly associated with obesity and various sleep health issues such as delayed sleep pattern, short sleep duration, insomnia and habitual snoring [157]. Besides, sleeping < 6 h
Epidemiology
Exposure to light at night may impair cognitive performances and induce excessive sleepiness and mood changes associated with the work schedule. Shift work has been associated with impaired cognition and the association was stronger for exposures lasting > 10 years [171]. Shift workers are at higher risks of fatigue, anxiety and depressive symptoms than day workers. This can result in absenteeism and a decline in work productivity [172], [173], [174].
A subsample of individuals (21–73 years old),
Phototoxicity
Age related macular degeneration (AMD), the leading cause of blindness in subjects older than 65 years of age, is a complex neurodegenerative visual disorder caused by the loss of retinal pigmented epithelium (RPE) cells and the light-sensitive photoreceptor cells that they support. The severe visual loss affects around 12% of the population of industrialized countries. Among the potential risk factors that have been documented in this multifactorial disorder, genetic factors are major with the
Methodological limitations of the studies
Some inconsistencies in results of the different studies devoted to the effects of ALAN on cancer risk are related to the fact that the definition of shift work and night work is itself different from a country to another which leads to differences in exposure classification and exposure contrast across studies and makes it difficult to compare and interpret results, and conduct meta-analyses [187]. Shift work and night work are characterized by a specific set of factors, and these should be
Preventive measures to combat circadian disruption and alleviate circadian misalignment
One of the long-term aims of researchers is to counter circadian misalignment and reduce its resulting deleterious effects on health. Testing has involved agents that are likely to resynchronize the clock, such as light and melatonin, as well as specific psychotropic medications with the aim of improving sleep, or conversely to enhance alertness, depending on the work requirements [188].
Conclusions and future perspectives
Approximately 75% of the active population in industrialized countries, work atypical hours; that is to say outside of the so-called normal let us say 08 h to 17 h business hours. By exposing workers to artificial light at night, shift and/or night work decreases the time spent sleeping, and it disrupts the circadian structure, the sleep cycle, social life, and meal times. This results in a perturbation of the functioning of the biological clock that is often called “social jet lag” because of
Conflict of interest
The authors declare that they have no conflict of interest.
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