Light regulates our internal clock

Light plays a crucial role in the regulation of our circadian rhythm (our internal biological clock) and consequently our sleep-wake cycle. More specifically, light is known as the primary zeitgeber, or time-teller, indicating to the brain when it is day-time and night-time. By using photic (light) information captured by the eyes, the brain is able to keep itself synchronized to its environment.

Cells of the retina of our eyes captures the photic information from our environment. For the most part, this information is then sent along the optic nerve to the visual cortex of the brain, located at the back of the head, allowing us to ‘see’ our environment.

However, in addition to this well known visual pathway, there is a subset of cells within the retina, named intrinsically photoreceptive retinal ganglion cells, that instead send the photic information from the eyes directly to the suprachiasmatic nucleus (SCN); the area of the brain that regulates our body’s circadian rhythm. Using the light information captured from our eyes, the SCN can align and maintain its internal clock to the environment’s day/night cycle.

The SCN controls, maintains and synchronizes a variety of biological rhythms through changes in cell and organ activity, nervous system activation, hormone regulation and gene expression. Among these biological rhythms is the regulation of our daily 24-hr sleep-wake cycle. This is done by both direct connections between the SCN and the sleep and wake promoting areas in the brain, as well as through indirect physiological changes and hormone secretion. For example, melatonin is our ‘night-time’ hormone that is produced and released to circulates in the body during the night, promoting a biological transition to sleep. In humans, melatonin is a biological indicator that it is night time and therefore sleep time.

The direct connections between the SCN and the pituary gland, where melatonin is produced, means that it is strongly influenced by our environmental light conditions. The natural decrease in the brightness of light come evening fall triggers the production of melatonin. On the other hand, bright lights attenuates melatonin production within our body. Thus, by being exposed to bright lights in the evening, our body cannot adequately produce melatonin to help us transition from wake to sleep.

Overall, by using light information, the internal biological clock ensures that we are awake and active during the day, and asleep and recovering throughout the night.

But what about blue light?
The specific cells in the retina that send information directly to the SCN for the regulation of our circadian rhythms responds to overall levels of brightness in our environment. As stated above, bright lights indicates to the brain that it is day time, while low levels of light indicates evening and night-time.

However, in addition to a slow and sustained response to overall brightness levels, these photoreceptive cells also have a greater affinity to the blue light spectrum (approx. 480 nm wavelength). Therefore, their response rate is enhanced when blue light is present.

This aligns well with the evolutionary fact that sunlight holds a high proportion of its light in the blue wavelength spectrum, while fire and candle light emits only low levels of blue wavelength light.

As such, optimizing exposure to bright and blue lights in the daytime and reducing their exposure in the evening and night-time will, consequently, help keep our circadian rhythm well synchronized to the 24-hr day/night cycle of our environment. Additionally, through manipulation of exposure, we can also shift internal clock to an earlier or later time, which has been shown to be beneficial shift and night workers as well as to reduce jet-lag symptoms after a long travel.

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Selected supporting references:

• Hankins, M. W., Peirson, S. N., & Foster, R. G. (2008). Melanopsin: an exciting photopigment. Trends in Neurosciences, 31(1), 27-36.
• Hattar, S., Liao, H.W., Takao, M., Berson, D.M., & Yau, K.W. (2002). Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science 295:1065-1070.
• Foster, R. G., & Hankins, M. W. (2007). Circadian vision. Current Biology, 17(17), R746-51.
• Skene, D. J., Lockley, S. W., Thapan, K., et al. (1999). Effects of light on human circadian rhythms. Reproduction Nutrition Development, 39(3), 295-304.
• Duffy, J.F. & Czeisler, C.A. (2009). Effect of light on human circadian physiology. Sleep Medicine Clinics 4: 165-177.
• Gooley, J.J., Chamberlain, K., Smith, K.A., Khalsa, S.B.S., Rajaratnam, S.M., Van Reen, E., Zeitzer, J.M., Czeisler, C.A., & Lockley, S.W. (2010). Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. Journal of Clinical Endocrinology & Metabolism, 96: E463-E472.