Man having a sleep in San Cristóbal, Cusco, Peru

A recent study published in Frontiers in Neuroscience has provided evidence that humans experience longer REM sleep in winter and less deep sleep in autumn, even in urban populations where sleep is commonly disrupted.

Dr Dieter Kunz, corresponding author of the study, based at the Clinic of Sleep & Chronomedicine at the St Hedwig Hospital, Berlin, said:

“Possibly one of the most precious achievements in human evolution is an almost invisibility of seasonality on the behavioral level. In our study we show that human sleep architecture varies substantially across seasons in an adult population living in an urban environment.”

Led by Ms. Aileen Seidler in Dr. Kunz’s working group at the Charité Medical University of Berlin, the study involved analyzing 292 patients who had undergone sleep studies over a year. Even though the patients were based in an urban environment with low natural light exposure and high light pollution, which should affect any seasonality regulated by light, the scientists found subtle but striking changes across the seasons. Although total sleep time appeared to be about an hour longer in the winter than in the summer, this result was not statistically significant. However, REM sleep was 30 minutes longer in the winter than summer. REM sleep is known to be directly linked to the circadian clock, which is affected by changing light. Although the team acknowledged that these results would need to be validated in a population that experiences no sleep difficulties, the seasonal changes may be even greater in a healthy population.

These findings may have implications for accommodating humans’ responses to the changing seasons. Although the team acknowledged the need for validation in a population that does not experience sleep difficulties, they suggest that the seasonal changes could be even more significant in a healthy population.

“Seasonality is ubiquitous in any living being on this planet. Even though we still perform unchanged, over the winter human physiology is down-regulated, with a sensation of ‘running-on-empty’ in February or March. In general, societies need to adjust sleep habits including length and timing to season, or adjust school and working schedules to seasonal sleep needs.”