Light as a central modulator of circadian rhythms, sleep and affect

Journal name: Nature Reviews Neuroscience Volume: 15, Pages: 443–454 Year published: DOI: doi:10.1038/nrn3743
Published online


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.

At a glance


  1. Model of the direct and indirect influences of light on mood and cognition.
    Figure 1: Model of the direct and indirect influences of light on mood and cognition.

    Light can regulate mood and learning by first modulating sleep and circadian rhythms (indirect pathway), or it can directly affect mood without disrupting sleep or causing circadian arrhythmicity (direct pathway). These pathways have also been shown to mediate the effects of light on hippocampal-dependent learning in rodents. The effects of light on circadian rhythms, sleep and mood are mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). Figure from Ref. 85, Nature Publishing Group.

  2. Retinal and brain circuits underlying the effects of light on non-image-forming visual functions.
    Figure 2: Retinal and brain circuits underlying the effects of light on non-image-forming visual functions.

    a | A schematic view of the retina showing the organization of different neuronal populations and their synaptic connections. Rods and cones are confined to the photoreceptor layer. Light detected by rods and cones is processed and signalled to retinal ganglion cells (RGCs) through horizontal, amacrine and bipolar cells. RGCs are the only output neurons from the retina to the brain. A subset of RGCs (4–5% of the total number of RCGs) are intrinsically photosensitive RGCs (ipRGCs). There are at least five subtypes of ipRGCs (M1–M5) with different morphological and electrophysiological properties, which show widespread projection patterns throughout the brain. b | ipRGCs project to numerous brain regions, including many that have a role in driving light-mediated behaviours, including circadian photoentrainment and sleep. In addition, ipRGCs also innervate nuclei involved in depression and/or anxiety, such as the medial amygdala (MA), lateral habenula (LHb) and subparaventricular zone (SPZ) (which are highlighted in green), indicating a possible direct role of light on mood. c | Several of the ipRGC targets, including the SPZ, ventrolateral preoptic area (VLPO), lateral hypothalamus (LH) and LHb, also receive innervation from the suprachiasmatic nucleus (SCN), raising the possibility that in addition to its pacemaker function, the SCN can also act as a conduit for light information. Interestingly, the MA and the LHb are also brain peripheral clocks (central and peripheral clocks are indicated by dashed lines) that receive direct retinal innervation. Areas involved in mood regulation (the ventral tegmental area (VTA) and raphe) and cognition (the hippocampus (HC)) can be influenced by light either through the SCN or in parallel through the MA and LHb. AH, anterior hypothalamus; BST, bed nucleus of the stria terminalis; IGL, intergeniculate leaflet; LC, locus coeruleus; LGNd, dorsal lateral geniculate nucleus; LGNv, ventral lateral geniculate nucleus; OPN, olivary pretectal nucleus; PAG, periaqueductal grey; pSON, supraoptic nucleus; SC, superior colliculus. Part a adapted with permission from Ref. 30, Elsevier. Part b adapted with permission from Ref. 131, Wiley.



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  1. Johns Hopkins University, Department of Biology, Baltimore, Maryland 21218, USA.

    • Tara A. LeGates,
    • Diego C. Fernandez &
    • Samer Hattar
  2. Present address: Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.

    • Tara A. LeGates
  3. Johns Hopkins University, Department of Neuroscience, Baltimore, Maryland 21218, USA.

    • Samer Hattar

Competing interests statement

The authors declare no competing interests.

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  • Tara A. LeGates

    Tara A. LeGates is currently a postdoctoral fellow in the laboratory of Scott Thompson in the Department of Physiology at the University of Maryland School of Medicine, Baltimore, USA. She received a B.S. in behavioural neuroscience at Rider University, Mercer County, New Jersey, USA. She received a Weintraub award for her Ph.D. thesis in the laboratory of Samer Hattar at the Department of Biology, Johns Hopkins University, Baltimore, Maryland, USA. Her research interests are to better understand the neural circuits underlying mood regulation and antidepressant responses.

  • Diego C. Fernandez

    Diego C. Fernandez is a postdoctoral fellow in the Department of Biology at the Johns Hopkins University, Baltimore, Maryland, USA, where he works with Samer Hattar. He earned his Ph.D. at Buenos Aires University in Argentina, and received a 2013 Pew Latin American postdoctoral fellowship. His research focuses on understanding the brain and retinal circuits that are involved in light effects on mood and cognitive functions.

  • Samer Hattar

    Samer Hattar is an associate professor at the Department of Biology at the Johns Hopkins University (Homewood campus), Baltimore, Maryland, USA, with a joint appointment in the Department of Neuroscience at the Johns Hopkins University School of Medicine. He received his B.S. from Yarmouk University in Irbid, Jordan and his Ph.D. from the University of Houston, Texas, USA. He carried out his postdoctoral studies in the Department of Neuroscience at the Johns Hopkins University School of Medicine. He is a Packard and Sloan scholar. His interests are how light modulates several behaviours, which include circadian rhythms, sleep, mood and learning. Samer Hattar's homepage.

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    Modelling aberrant light environments in the lab using rodents.

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