Extreme light conditions, as experienced in shift-work schedules, shortened day length during winter months or transmeridian travel, can cause negative health effects, which include cognitive deficits and mood alterations.
Aberrant light schedules can cause circadian rhythm alterations and/or sleep disruptions, leading to mood disorders and depression-like states. This has led to a model in which the negative effects of irregular light on mood and cognitive functions are secondary to circadian and/or sleep disruptions.
However, light can also directly influence mood and learning without causing circadian arrhythmicity or sleep disruptions.
Intrinsically photosensitive retinal ganglion cells (ipRGCs) project to a wide range of brain regions, including areas that have been associated with mood and anxiety. These cells emerge as leading candidates for mediating the direct effects of irregular light on mood.
Rodent models have been used to define the mechanisms involved in the effects of light on circadian rhythms, sleep and mood. Several animal models and light schedules were used to recapitulate important aspects of irregular light schedules observed in humans, which cause mood and cognitive disorders.
A better understanding of the connections that ipRGCs form in the brain and their influence on circadian, sleep and mood controlling centres will be the first step to determine the comprehensive role of light on these complex, inter-related behaviours.
Light has profoundly influenced the evolution of life on earth. As widely appreciated, light enables us to generate images of our environment. However, light — through intrinsically photosensitive retinal ganglion cells (ipRGCs) — also influences behaviours that are essential for our health and quality of life but are independent of image formation. These include the synchronization of the circadian clock to the solar day, tracking of seasonal changes and the regulation of sleep. Irregular light environments lead to problems in circadian rhythms and sleep, which eventually cause mood and learning deficits. Recently, it was found that irregular light can also directly affect mood and learning without producing major disruptions in circadian rhythms and sleep. In this Review, we discuss the indirect and direct influence of light on mood and learning, and provide a model for how light, the circadian clock and sleep interact to influence mood and cognitive functions.
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The authors acknowledge H. Zhao, L. Ospri and M. Kvarta for the careful reading of and suggestions for this Review.
The authors declare no competing financial interests.
- Non-image-forming visual functions
(NIF visual functions). Light mediated-behaviours that are not involved in detecting contrasts, colour or motion of the visual scene. These include circadian photoentrainment and sleep regulation.
- Circadian photoentrainment
The synchronization of internal circadian rhythms with the solar cycle. This renders circadian rhythms physiologically relevant.
- Sleep drive
The pressure to sleep. Sleep drive is driven by homeostatic and circadian mechanisms that interact to determine its strength. The higher the sleep drive, the easier it is to fall asleep.
- Shift work
A work schedule that varies irrespective of the solar day–night cycles. This type of work causes sleep and circadian problems and induces major health issues.
- Seasonal affective disorder
(SAD). A seasonal form of depression that occurs as a manifestation of the shorter day length of the winter months.
- Circadian rhythms
Internally generated near 24-hour rhythms in biological processes that are expressed in almost all tissues throughout the body.
- Suprachiasmatic nucleus
(SCN). A brain region located in the hypothalamus that houses the central circadian pacemaker. The SCN synchronizes peripheral rhythms with the solar cycle.
Hormones produced by the adrenal cortex that are involved in carbohydrate and protein metabolism and also affect brain function. Cortisol (human) and corticosterone (rodent) are prime examples.
- Jet lag
A syndrome that occurs upon crossing different time zones through transmeridian travel.
- Long-term potentiation
A lasting increase in synaptic transmission in response to a strong correlated input.
- Peripheral oscillators
Tissues, apart from the suprachiasmatic nucleus, that are capable of circadian rhythm generation.
- Forced swim test
A test in which mice are placed in a water tank to measure depression-like states. Most depressed mice eventually develop an immobile posture, which is thought to indicate behavioural despair.
Natural variations in a gene or a particular DNA region. These genotypic differences give rise to more than one morph, evidenced as differences in the phenotype.
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LeGates, T., Fernandez, D. & Hattar, S. Light as a central modulator of circadian rhythms, sleep and affect. Nat Rev Neurosci 15, 443–454 (2014). https://doi.org/10.1038/nrn3743
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