Light effects on circadian rhythm

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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.[1] In order to maintain synchronization ("entrainment") to 24 hours, external factors must play some role. The reason why entrainment occurs in humans is because each individual's circadian rhythm is longer than 24 hours (majority of population) or shorter than 24 hours (minority of population).[2][3] Of the various factors that influence this entrainment, light exposure to the eyes is the strongest.[4][5][6] Melatonin plays a large role in effects of light on circadian rhythms.[2][7] When an organism is exposed to a light stimulus, the hormone melatonin is suppressed, or prevented from being secreted by the pineal gland.[2]

Demonstrated effects

All of the mechanisms of light-effected entrainment are not yet fully known, however numerous studies have demonstrated the effectiveness of light entrainment to the day/night cycle. Studies have shown that the timing of exposure to light influences entrainment; as seen on the phase response curve for light for a given species. In diurnal (day-active) species, exposure to light soon after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm.[6][8][9] An advance means that the individual will tend to wake up earlier on the following day(s). A delay, caused by light exposure before sleeping, means that the individual will tend to wake up later on the following day(s).

The length of light exposure influences entrainment. Longer exposures have a greater effect than shorter exposures.[8] Consistent light exposure has a greater effect than intermittent exposure.[4] In rats, constant light eventually disrupts the cycle to the point that memory and stress coping may be impaired.[10]

The intensity and the wavelength of light influence entrainment.[2] Dim light can affect entrainment relative to darkness.[11] Brighter light is more effective than dim light.[8] In humans, a lower intensity short wavelength (blue/violet) light appears to be equally effective as a higher intensity of white light.[6]

Internal pathways and mechanisms

Light first passes into a mammal's system through the retina, then takes one of two paths: the light either gets collected by rod cells and cone cells before being transmitted to the retinal ganglion cells (RGCs), or it is directly collected by these RGCs.[2][3][12][13] The RGCs use the photopigment melanopsin to absorb the light energy.[2][3][12][13] Specifically, this class of RGCs being discussed is referred to as "intrinsically photosensitive," which just means they are sensitive to light.[2][7][12] There are five known types of intrinsically photosensitive retinal ganglion cells (ipRGCs): M1, M2, M3, M4, and M5.[12] These connect to amacrine cells in the inner plexiform layer of the retina.[12] Ultimately, via this retinohypothalamic tract (RHT) the suprachiasmatic nucleus (SCN) of the hypothalamus receives light information from these ipRCGs.[2][3][12][13]

The core region of the SCN houses the majority of light-sensitive neurons.[9] From here, signals must reach the rest of the SCN in order for circadian shifts, or entrainment, to occur.[9]

There are specific genes that determine the regulation of circadian rhythm in conjunction with light.[9] When light activates NMDA receptors in the SCN, CLOCK gene expression in that region is altered and the SCN is reset, and this is how entrainment occurs.[9] Genes also involved with entrainment are PER1 and PER2.[9]

Some important structures directly impacted by the light-sleep relationship are the superior colliculus-pretectal area and the ventrolateral pre-optic nucleus.[7][13]

Secondary effects

While light has direct effects on circadian rhythm, there are indirect effects seen across studies.[12] Seasonal affective disorder creates a model in which decreased day length during autumn and winter increases depressive symptoms.[7][12] A shift in the circadian phase response curve creates a connection between the amount of light in a day (day length) and depressive symptoms in this disorder.[7][12] Light seems to have therapeutic antidepressant effects when an organism is exposed to it at appropriate times during the circadian rhythm, regulating the sleep-wake cycle.[7][12]

In addition to mood, learning and memory become impaired when the circadian system shifts due to light stimuli,[7][14] which can be seen in studies modeling jet lag and shift work situations.[12] Frontal and parietal lobe areas involved in working memory have been implicated in melanopsin responses to light information.[14]

In response to light exposure, alertness levels can increase as a result of suppression of melatonin secretion.[3][7] A linear relationship has been found between alerting effects of light and activation in the posterior hypothalamus[3][15].

Disruption of circadian rhythm as a result of light also produces changes in metabolism.[12]

Other factors

Although many researchers consider light to be the strongest cue for entrainment, it is not the only factor acting on circadian rhythms. Other factors may enhance or decrease the effectiveness of entrainment. For instance, exercise and other physical activity, when coupled with light exposure, results in a somewhat stronger entrainment response.[4] Other factors such as music and properly timed administration of the neurohormone melatonin have shown similar effects.[16][17] Numerous other factors affect entrainment as well. These include feeding schedules, temperature, pharmacology, locomotor stimuli, social interaction, sexual stimuli and stress.[18]


Dark therapy before bedtime, where light that triggers melanopsin receptors is filtered out with eyewear, is considered an effective method to promote earlier production of melatonin and improve sleep.[citation needed]

See also


  1. ^ Kolmos, E. and S.J. Davis (2007). "Circadian rhythms: Rho-related signals in time-specific light perception." Current Biology 17(18): R808–R810.
  2. ^ a b c d e f g h Duffy, Jeanne F.; Czeisler, Charles A. (2009). "Effect of Light on Human Circadian Physiology". Sleep Medicine Clinics. 4 (2): 165–177. doi:10.1016/j.jsmc.2009.01.004. PMC 2717723. PMID 20161220.
  3. ^ a b c d e f Vimal, Ram L. P.; Pandey-Vimal, Manju-Uma C.; Vimal, Love-Shyam P.; Frederick, Blaise B.; Stopa, Edward G.; Renshaw, Perry F.; Vimal, Shalini P.; Harper, David G. (2009-01-01). "Activation of suprachiasmatic nuclei and primary visual cortex depends upon time of day". European Journal of Neuroscience. 29 (2): 399–410. doi:10.1111/j.1460-9568.2008.06582.x. ISSN 1460-9568. PMID 19200242.
  4. ^ a b c Baehr, E.K., L.F. Fogg, et al. (1999). "Intermittent bright light and exercise to entrain human circadian rhythms to night work." American Journal of Physiology. Regulatory Integrative and Comparative Physiology 277(6): R1598–R1604.
  5. ^ Hirayama, J., S. Cho, et al. (2007). "Circadian control by the reduction/oxidation pathway: Catalase represses light-dependent clock gene expression in the zebrafish." Proceedings of the National Academy of Sciences of the United States of America 104(40): 15747–15752.
  6. ^ a b c Warman, V.L., D.J. Dijk, et al. (2003). "Phase advancing human circadian rhythms with short wavelength light." Neuroscience Letters 342(1–2): 37–40.
  7. ^ a b c d e f g h Stephenson, Kathryn M.; Schroder, Carmen M.; Bertschy, Gilles; Bourgin, Patrice (2012). "Complex interaction of circadian and non-circadian effects of light on mood: Shedding new light on an old story". Sleep Medicine Reviews. 16 (5): 445–454. doi:10.1016/j.smrv.2011.09.002.
  8. ^ a b c Duffy, J.F., R.E. Kronauer, et al. (1996). "Phase-shifting human circadian rhythms: Influence of sleep timing, social contact and light exposure." Journal of Physiology-London 495(1): 289–297.
  9. ^ a b c d e f Yan, Lily (2009-09-24). "Expression of clock genes in the suprachiasmatic nucleus: Effect of environmental lighting conditions". Reviews in Endocrine and Metabolic Disorders. 10 (4): 301–310. doi:10.1007/s11154-009-9121-9. ISSN 1389-9155.
  10. ^ Ma, W.P., J. Cao, et al. (2007). "Exposure to chronic constant light impairs spatial memory and influences long-term depression in rats." Neuroscience Research 59(2): 224–230.
  11. ^ Gorman, M.R., M. Kendall, et al. (2005). "Scotopic illumination enhances entrainment of circadian rhythms to lengthening Light : Dark cycles." Journal of Biological Rhythms 20(1): 38–48.
  12. ^ a b c d e f g h i j k l LeGates, Tara A.; Fernandez, Diego C.; Hattar, Samer (2014). "Light as a central modulator of circadian rhythms, sleep and affect". Nature Reviews Neuroscience. 15 (7): 443–454. doi:10.1038/nrn3743. PMC 4254760. PMID 24917305.
  13. ^ a b c d Dijk, Derk-Jan; Archer, Simon N. (2009-06-23). "Light, Sleep, and Circadian Rhythms: Together Again". PLoS Biol. 7 (6): e1000145. doi:10.1371/journal.pbio.1000145. PMC 2691600. PMID 19547745.
  14. ^ a b Vandewalle, G.; Gais, S.; Schabus, M.; Balteau, E.; Carrier, J.; Darsaud, A.; Sterpenich, V.; Albouy, G.; Dijk, D. J. (2007-12-01). "Wavelength-Dependent Modulation of Brain Responses to a Working Memory Task by Daytime Light Exposure". Cerebral Cortex. 17 (12): 2788–2795. doi:10.1093/cercor/bhm007. ISSN 1047-3211. PMID 17404390.
  15. ^ Vandewalle, Gilles; Balteau, Evelyne; Phillips, Christophe; Degueldre, Christian; Moreau, Vincent; Sterpenich, Virginie; Albouy, Geneviève; Darsaud, Annabelle; Desseilles, Martin (2006). "Daytime Light Exposure Dynamically Enhances Brain Responses". Current Biology. 16 (16): 1616–1621. doi:10.1016/j.cub.2006.06.031. ISSN 0960-9822. PMID 16920622.
  16. ^ Goel, N. (2006). "An arousing, musically enhanced bird song stimulus mediates circadian rhythm phase advances in dim light." American Journal of Physiology. Regulatory Integrative and Comparative Physiology 291(3): R822–R827.
  17. ^ Revell, V.L., H.J. Burgess, et al. (2006). "Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light." Journal of Clinical Endocrinology and Metabolism 91(1): 54–59.
  18. ^ Salazar-Juarez, A., L. Parra-Gamez, et al. (2007). "Non-photic entrainment. Another type of entrainment? Part one." Salud Mental 30(3): 39–47.
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