Light effects on circadian rhythm and sleep mask

Light effects on circadian rhythm (circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats every 24 hours) and sleep mask

It can refer to any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.

Light effects on circadian rhythm

Light effects on circadian rhythm are the effects that light has on circadian rhythm, sleep, mood, Life.

Light effects on circadian rhythm and sleep mask
Light effects on circadian rhythm and sleep mask

Most animals and other organisms have “built-in clocks” in their brains that regulate the timing of biological processes and daily behavior. These “clocks” are known as circadian rhythms.

They allow maintenance of these processes and behaviors relative to the 24-hour day/night cycle in nature. Although these rhythms are maintained by the individual organisms, their length does vary somewhat individually.

Therefore, they must, either continually or repeatedly, be reset to synchronize with nature’s cycle. In order to maintain synchronization to 24 hours, external factors must play some role. The human circadian rhythm occurs typically in accordance with nature’s cycle. The average activity rhythm cycle is 24.18 hours in adulthood but is shortened as age increases.

One of the various factors that influence this entrainment is light exposure to the eyes. When an organism is exposed to a specific wavelength of light stimulus at certain times throughout the day, the hormone melatonin is suppressed, or prevented from being secreted by the pineal gland.

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Effects of light on human circadian rhythms, sleep and mood


live in a 24-hour environment, in which light and darkness follow
a diurnal pattern. Our circadian pacemaker, the suprachiasmatic nuclei
(SCN) in the hypothalamus, is entrained to the 24-hour solar day via
a pathway from the retina and synchronises our internal biological
rhythms. Rhythmic variations in ambient illumination impact behaviours
such as rest during sleep and activity during wakefulness as well as
their underlying biological processes. Rather recently, the availability
of artificial light has substantially changed the light environment,
especially during evening and night hours. This may increase the risk of
developing circadian rhythm sleep–wake disorders (CRSWD), which are
often caused by a misalignment of endogenous circadian rhythms and
external light–dark cycles. While the exact relationship between the
availability of artificial light and CRSWD remains to be established,
nocturnal light has been shown to alter circadian rhythms and sleep in
humans. On the other hand, light can also be used as an effective and
noninvasive therapeutic option with little to no side effects, to
improve sleep, mood and general well-being. This article reviews our
current state of knowledge regarding the effects of light on circadian
rhythms, sleep, and mood.


Circadian rhythms, Natural light, Artificial light Depression, Light therapy 


ADHD – Attention deficit hyperactivity disorder

BLT – Bright light therapy

CCT – Correlated colour temperature

CIE – Commission Internationale de l’Eclairage

CRSWD – Circadian rhythm sleep-wake disorders

DLMO – Dim-light melatonin onset

EEG – Electroencephalogram

GHT – Geniculohypothalamic tract

IGL – Intergeniculate leafletip

RGC – Intrinsically photosensitive retinal ganglion cell

LED – Light-emitting diode

PRC – Phase response curve

RGC – Retinal ganglion cell

RHT – Retinohypothalamic tract

RN – Raphe nuclei

SAD – Seasonal affective disorder

SCN – Suprachiasmatic nuclei

SSRI – Selective serotonin reuptake inhibitor

SWA – Slow wave activity

UV – Ultraviolet

All authors contributed equally to the manuscript.

Auswirkungen von Licht auf zirkadiane Rhythmen, Schlaf und die Stimmung bei Menschen


Mensch lebt in einer 24-Stunden-Umgebung, in der sich Licht und
Dunkelheit abwechseln. Unser zirkadianer Schrittmacher in den
suprachiasmatischen Nuclei (SCN) des Hypothalamus synchronisiert unsere
inneren biologischen Rhythmen mit dem Tagesverlauf des Sonnenlichts über
Leitungsbahnen, die von der Netzhaut zu den SCN ziehen. Rhythmische
Schwankungen in der Umgebungsbeleuchtung beeinflussen unser Verhalten,
etwa den Wechsel zwischen Ruhe im Schlaf und Aktivität im Wachzustand,
und dessen zugrunde liegenden biologischen Prozesse. In jüngster Zeit
hat die Verfügbarkeit von künstlichem Licht die Lichtumgebung erheblich
verändert, insbesondere während der Abend- und Nachtstunden. Dies kann
das Risiko für die Entstehung von circadianen
Schlaf-Wach-Rhythmusstörungen („circadian rhythm sleep-wake disorders“,
CRSWD) erhöhen, die oft durch eine Fehlausrichtung bzw. einen Versatz
zwischen endogenen tageszeitlichen Rhythmen und dem externen
Hell-Dunkel-Zyklus verursacht werden. Zwar ist der genaue Zusammenhang
zwischen der Verfügbarkeit von künstlichem Licht und CRSWD noch nicht
geklärt, doch es hat sich gezeigt, dass nächtliches Licht den
zirkadianen Rhythmus und den Schlaf beim Menschen verändert.
Andererseits kann Licht auch als effektive, nichtinvasive Therapieoption
mit geringen bis keinen Nebenwirkungen eingesetzt werden, um Schlaf,
Stimmung und Allgemeinbefinden zu verbessern. Der vorliegende Beitrag
gibt einen Überblick über den aktuellen Wissensstand hinsichtlich der
Auswirkungen von Licht auf zirkadiane Rhythmen, Schlaf und Stimmung.


Zirkadiane Rhythmen, Natürliches Licht, Künstliches Licht Depression, Lichttherapie 

Anatomical architecture of the circadian system

The central master-clock in mammalian species, including humans, is the suprachiasmatic nuclei (SCN), a paired structure in the hypothalamus with a volume just about 0.25 mm3 per nucleus (e.g. [45, 57, 84]). Within the mammalian SCN, a molecular oscillator keeps the clock oscillating at its normal pace. The basis of this oscillator is two interconnected molecular feedback loops of clock gene expression, a detailed description of which is beyond the scope of this review though (see [12] for a detailed explanation).


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Light effects on circadian rhythm and sleep mask

Systematic review of light exposure impact on human circadian rhythm


Light is necessary for life, and artificial light improves visual performance and safety, but there is an increasing concern of the potential health and environmental impacts of light. Findings from a number of studies suggest that mistimed light exposure disrupts the circadian rhythm in humans, potentially causing further health impacts. However, a variety of methods has been applied in individual experimental studies of light-induced circadian impacts, including definition of light exposure and outcomes. Thus, a systematic review is needed to synthesize the results. In addition, a review of the scientific evidence on the impacts of light on circadian rhythm is needed for developing an evaluation method of light pollution, i.e., the negative impacts of artificial light, in life cycle assessment (LCA). The current LCA practice does not have a method to evaluate the light pollution, neither in terms of human health nor the ecological impacts. The systematic literature survey was conducted by searching for two concepts: light and circadian rhythm. The circadian rhythm was searched with additional terms of melatonin and rapid-eye-movement (REM) sleep. The literature search resulted to 128 articles which were subjected to a data collection and analysis. Melatonin secretion was studied in 122 articles and REM sleep in 13 articles. The reports on melatonin secretion were divided into studies with specific light exposure (101 reports), usually in a controlled laboratory environment, and studies of prevailing light conditions typical at home or work environments (21 studies). Studies were generally conducted on adults in their twenties or thirties, but only very few studies experimented on children and elderly adults. Surprisingly many studies were conducted with a small sample size: 39 out of 128 studies were conducted with 10 or less subjects. The quality criteria of studies for more profound synthesis were a minimum sample size of 20 subjects and providing details of the light exposure (spectrum or wavelength; illuminance, irradiance or photon density). This resulted to 13 qualified studies on melatonin and 2 studies on REM sleep. Further analysis of these 15 reports indicated that a two-hour exposure to blue light (460 nm) in the evening suppresses melatonin, the maximum melatonin-suppressing effect being achieved at the shortest wavelengths (424 nm, violet). The melatonin concentration recovered rather rapidly, within 15 min from cessation of the exposure, suggesting a short-term or simultaneous impact of light exposure on the melatonin secretion. Melatonin secretion and suppression were reduced with age, but the light-induced circadian phase advance was not impaired with age. Light exposure in the evening, at night and in the morning affected the circadian phase of melatonin levels. In addition, even the longest wavelengths (631 nm, red) and intermittent light exposures induced circadian resetting responses, and exposure to low light levels (5–10 lux) at night when sleeping with eyes closed induced a circadian response. The review enables further development of an evaluation method of light pollution in LCA regarding the light-induced impacts on human circadian system.

KEYWORDS: Light, human health, melatonin, REM sleep, health, circadian rhythm


Artificial light is a necessity in the modern societies. It provides illumination when natural light is not available enabling a multitude of functions after dark. It is intended to improve the visual performance and visibility in indoor and outdoor environments, thus improving safety and comfort of humans. In addition, artificial light may serve as a source for relaxation and beauty, and to improve alertness and productivity.


The human circadian system in normal and disordered sleep.


The human circadian system regulates rhythmicity in the human body and establishes normal sleep and wake phases. The suprachiasmatic nuclei (SCN), located in the hypothalamus above the optic chiasm, make up the human pacemaker known as the circadian or biological clock, but other essential parts of the circadian system include the pineal gland, retina, and retinohypothalamic tract. The importance of light in resetting the intrinsic human circadian cycle, the intrinsic period of which is slightly longer than 24 hours, ensures that the human cycle will stay entrained to the earth’s 24-hour daily cycle. Within the SCN neurons, circadian rhythmicity is generated by the regular transcription of proteins. Since the circadian system is the foundation of the sleep-wake cycle, disorders and abnormalities in sleep are often connected with disorders or abnormalities in the circadian system. Circadian rhythm sleep disorders, such as jet lag syndrome and shift work sleep disorder, are those specifically attributed to dysfunctions or insufficiencies in the circadian system. Taking into consideration the preeminence of the circadian clock in timing sleep, it is likely that other sleep disorders, such as insomnia, are also linked to circadian system abnormalities.


Effect of Light on Human Circadian Physiology


The circadian system in animals and humans, being near but not exactly 24-hours in cycle length, must be reset on a daily basis in order to remain in synchrony with external environmental time. This process of entrainment is achieved in most mammals through regular exposure to light and darkness. In this chapter, we review the results of studies conducted in our laboratory and others over the past 25 years in which the effects of light on the human circadian timing system were investigated. These studies have revealed, how the timing, intensity, duration, and wavelength of light affect the human biological clock. Our most recent studies also demonstrate that there is much yet to learn about the effects of light on the human circadian timing system.

Keywords: biological rhythm, core body temperature, illuminance, melatonin, phase response curve


rhythms are variations in physiology and behavior that persist with a
cycle length close to 24 hours even in the absence of periodic
environmental stimuli. It is hypothesized that this system evolved in
order to predict and therefore optimally time the behavior and
physiology of the organism to the environmental periodicity associated
with the earth’s rotation. Because the cycle length, or period, of this
endogenous timing system is near, but, in most organisms, not exactly 24
hours, circadian rhythms must be synchronized or entrained to the
24-hour day on a regular basis. In most organisms, this process of
entrainment occurs through regular exposure to light and darkness.

Early reports from studies of human circadian rhythms had suggested that humans were unlike other organisms, being relatively insensitive to light and more sensitive to social cues to entrain their circadian systems. However, subsequent studies, and re-analysis of results from those early studies, have found that the human circadian system is like that of other organisms in its organization and its response to light, and is as sensitive to light as other diurnal organisms. In this chapter we review the results of studies conducted in our laboratory and others over the past 25 years in which the effects of light on the human circadian timing system were investigated.


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Light effects on circadian rhythm and sleep mask

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