With an increasingly sun-safe and sedentary society, people aren’t exposed to sunlight like they used to be. Instead of ploughing the fields or kicking a ball outside we spend all our days in offices under artificial light and our nights in front of the TV. With the technological advancements of the 21st century average sleep duration has declined while sleep disorder diagnoses have steadily risen. Research suggests this is no coincidence.
The suprachiasmatic nucleus (SCN) is the pacemaker of circadian rhythm and controls sleep and wake cycles by stimulating the pineal gland to synthesise the sleep hormone melatonin. The activities of the SCN itself is modulated by the amount and quality of light received by cells in the retina of the eye, with daylight inhibiting melatonin production and stimulating wakefulness. Seasonal changes in daytime light intensity and duration results in a variation in circadian rhythm between the winter and summer months. This has been associated with increased daytime sleepiness, seasonal depression and later sleep and wake times. According to a recent study by researchers at the University of Washington there may be a simple solution to this issue.
To investigate this phenomenon light exposure and sleep times were measured via actigraphy watch in 500 university students over the course of this year. Expectedly, daytime light exposure was significantly greater in the summer months with an average of 4.6 hours more exposure than in the winter. Meanwhile, night-time light exposure was 4.3 hours greater in the Winter. This was reflected in average sleep onset and offset times. Sleep onset in the winter was delayed by an average of 35 minutes while offset was delayed by 27 minutes compared to the summer months. Alarm use also differed between the seasons with 61% using alarms during winter compared to only 53% in the summer.
These findings identify two main culprits responsible for the shift in circadian rhythm during the winter: decreased daytime and increased night-time light exposure. This delays wakefulness in the morning whilst stimulating it in the evenings. In the words of the papers’ senior author Horacio de la Iglesias:
“What this study shows is that we need to get out — even for a little while and especially in the morning — to get that natural light exposure. In the evening, minimise screen time and artificial lighting to help us fall asleep.”
For more information this research read an article on the paper at https://www.sciencedaily.com/releases/2022/12/221212140707.htm or read the original paper at https://doi.org/10.1111/jpi.12843
(Review of ‘Daytime light exposure is a strong predictor of seasonal variation in sleep and circadian timing of university students’ by Dunster et al., published in the Journal of Pineal Research on the 20th of November 2022).
