Let there be light
Light is essential for humans; it adjusts the circadian rhythm and impact on many physiological and behavioural activities such as sleep, mood, neuro-endocrine function and cognition. Increased daytime light improves alertness, mood, performance and sleep and decreased light exposure in the evening and at night can promote sleep. It is essential to keep some regularity in the intensity, timing and length of light exposure in order to not alter the biological rhythms with negative impact on sleep, health, cognition and judgments.1,2
It is thought to be important to ensure you are exposed to different wavelengths and intensities of light throughout the day to help maintain your circadian rhythm. One of the key moments is within the first 2 hours of waking, aim to get natural light into your eyes, without sunglasses, and don't look directly into the sun. The amount of time depends on cloud cover, it should be between 10-20 minutes. This triggers the timed release of a healthy level of cortisol into your system which acts as a signal to your body to wake-up boosting focus and alertness throughout the day. 3,4,5
Modern humans are more often exposed to artificial light – resulting in less light during the day, while electric lightning during the night reduces or even removes darkness. The direction of light is also important as overhead lighting can activate receptor pigments (melanopsin etc.) located in the bottom of the eye de-synchronising the circadian rhythm. Late evening light exposure inhibits melatonin release, reduces sleepiness and delays the circadian clock. Even low levels of light in the sleeping environment have been associated to impaired sleep and metabolic dysfunctions. 3,4
Light is captured in the eye by photopigments. One of these pigments, melanopsin, is sensitive to a portion of the light spectrum, peaking in the blue colour. When melanopsin is activated it conveys information to the master biological clock in the brain (the suprachiasmatic nucleus), synchronizing circadian rhythms and regulating melatonin release. Melanopsin receptors have their own circadian rhythm they are particularly sensitive during evening and nighttime hours – explaining why exposure to even small amounts of blue light at night can have an important effect in destabilizing the circadian system. Increases in LED lightning in streets and buildings (sometimes more than 50% in some European cities) with increased blue light radiation increases suppression of melatonin regulating sleep.1,2,3,4,6
Red light therapy is a term that uses both red light, with a wavelength of 600-700nm, and near infrared (NIR) that has a wavelength of 700 to 1,100nm. Both of these bands of wavelength are thought to act upon the mitochondria to stimulate ATP (the energy molecule) production. Although this would seem counter to sleep, red and NIR light has been shown to increase the production of melatonin outside of its traditional synthesis within the pineal gland. It is thought that this is related to melatonin's role in helping to protect the mitochondria from radical oxygen species.1,2
Red light has also been shown to help reset the circadian rhythm alongside its ability to increase melatonin synthesis and promote sleep.3,4
[1.] Yin, J., Julius, A. A., & Wen, J. T. (2021). Optimization of light exposure and sleep schedule for circadian rhythm entrainment. PLoS ONE, 16(6 June).
[2.] Böhmer, M. N., Hamers, P. C. M., Bindels, P. J. E., Oppewal, A., van Someren, E. J. W., & Festen, D. A. M. (2021). Are we still in the dark? A systematic review on personal daily light exposure, sleep-wake rhythm, and mood in healthy adults from the general population. In Sleep Health (Vol. 7, Issue 5).
[3.] Figueiro, M. G., Steverson, B., Heerwagen, J., Kampschroer, K., Hunter, C. M., Gonzales, K., Plitnick, B., & Rea, M. S. (2017). The impact of daytime light exposures on sleep and mood in office workers. Sleep Health, 3(3).
[4.] Marqueze, E. C., Vasconcelos, S., Garefelt, J., Skene, D. J., Moreno, C. R., & Lowden, A. (2015). Natural light exposure, sleep and depression among day workers and shiftworkers at Arctic and Equatorial Latitudes. PLoS ONE, 10(4).
[5.] Mason, I. C., Grimaldi, D., Reid, K. J., Warlick, C. D., Malkani, R. G., Abbott, S. M., & Zee, P. C. (2022). Light exposure during sleep impairs cardiometabolic function. Proceedings of the National Academy of Sciences of the United States of America, 119(12).
[6.] Preciado, O. U., Issolio, L. A., Manzano, E., Colombo, E., & Barrionuevo, P. A. (2018). Melanopsin excitation in conditions of natural and artificial lighting. Anales de La Asociacion Fisica Argentina, 29(Especial InVision).
[2.] Böhmer, M. N., Hamers, P. C. M., Bindels, P. J. E., Oppewal, A., van Someren, E. J. W., & Festen, D. A. M. (2021). Are we still in the dark? A systematic review on personal daily light exposure, sleep-wake rhythm, and mood in healthy adults from the general population. In Sleep Health (Vol. 7, Issue 5).
[3.] Figueiro, M. G., Steverson, B., Heerwagen, J., Kampschroer, K., Hunter, C. M., Gonzales, K., Plitnick, B., & Rea, M. S. (2017). The impact of daytime light exposures on sleep and mood in office workers. Sleep Health, 3(3).
[4.] Marqueze, E. C., Vasconcelos, S., Garefelt, J., Skene, D. J., Moreno, C. R., & Lowden, A. (2015). Natural light exposure, sleep and depression among day workers and shiftworkers at Arctic and Equatorial Latitudes. PLoS ONE, 10(4).
[5.] Mason, I. C., Grimaldi, D., Reid, K. J., Warlick, C. D., Malkani, R. G., Abbott, S. M., & Zee, P. C. (2022). Light exposure during sleep impairs cardiometabolic function. Proceedings of the National Academy of Sciences of the United States of America, 119(12).
[6.] Preciado, O. U., Issolio, L. A., Manzano, E., Colombo, E., & Barrionuevo, P. A. (2018). Melanopsin excitation in conditions of natural and artificial lighting. Anales de La Asociacion Fisica Argentina, 29(Especial InVision).