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How does light influence sleep?

Article written by Rut Ruiz, scientific editor at AdSalutem Instituto del sueño, a partner of the BBHI in the field of sleep.

Sun exposure is essential for quality sleep and to protect our health. It is the main environmental signal for regulating our circadian rhythms, allowing us to synchronise our biological clock with the daily and seasonal light-dark cycle. So it is just as important to receive sufficient light during the day as it is to receive darkness at night.

Most physiological processes are influenced by these circadian oscillations: temperature control, blood pressure, hormone regulation; or some of our behaviours (when to sleep or wake up, when to exercise, eat or be more productive and creative). Circadian dysfunctions are implicated in some of the most common sleep disorders, for example, jet lag or phase delay, characterised by a later sleep schedule.

The quality of sleep depends directly on how synchronised our clock is with the light-dark cycle, as well as the nuances of colour, intensity and timing of light exposure.

Waking up to bright light at the start of the day is beneficial for activation and subsequent sleep quality. In this respect, the recent change to winter time helps us to coincide the start of our activity with sunrise.

However, exposure to bright, intense blue light from screens late in the day is detrimental to sleep. It is no coincidence that evening light is warm, orange and less intense, as it signals that we should prepare for sleep. Darkness is the signal for the synthesis of melatonin – a hormone involved in the sleep process – which serves as a messenger of darkness to the rest of the body. Conversely, cortisol, the hormone that allows us to be alert, is at its lowest levels when darkness arrives, and highest shortly after waking up.

 

Vitamin D and sleep quality

On the other hand, sunlight is also essential for the synthesis and conversion of Vitamin D, which is involved in the maintenance of bones, muscles and the function of the immune system. Therefore, a deficiency of this vitamin – which is common in northern hemisphere countries such as Spain – can be a problem for our health. Recent studies have also linked vitamin D deficiency to loss of sleep quality and daytime sleepiness.

Vitamin D synthesis is strongly influenced by the season, time of day, latitude and altitude. During the winter months, the solar zenith angle increases, which reduces the amount of UVB radiation reaching the earth’s surface and consequently decreases vitamin D production.

In short, we are biological organisms subject to circadian oscillations, so we must adapt our habits to the daily and seasonal light-dark cycle. Otherwise, we could alter our biological rhythms and affect our health.

 

References:

Leon LACK, Helen WRIGHT, Damien PAYNTER, The treatment of sleep onset insomnia with bright morning light, Sleep and Biological Rhythms, 10.1111/j.1479-8425.2007.00272.x, 5, 3, (173-179), (2007).

Wacker, M., & Holick, M. F. (2013). Sunlight and Vitamin D: A global perspective for health. Dermato-endocrinology, 5(1), 51–108. https://doi.org/10.4161/derm.24494

Majid MS, Ahmad HS, Bizhan H, Hosein HZM, Mohammad A. The effect of vitamin D supplement on the score and quality of sleep in 20-50 year-old people with sleep disorders compared with control group. Nutr Neurosci. 2018 Sep;21(7):511-519. doi: 10.1080/1028415X.2017.1317395. Epub 2017 May 5. PMID: 28475473.

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