Seasonal changes may mean we need more sleep in winter

Whilst humans don’t hibernate, a new study suggests seasonal sleep cycle variation means we require more sleep in the winter months.

Do you find yourself hitting snooze more in the winter? Or start February days feeling more drained than in June or July? Researchers at St. Hedwig Hospital and Charité Medical University of Berlin (both Berlin, Germany) have found that patients with sleep-related difficulties show seasonal variations in the length and depth of their sleep. The authors suggest that if the findings are true for healthy populations, it may indicate a societal need to adapt to our need of more sleep in the winter.

Previous studies have used self-reported information to study seasonal patterns in sleep and found differences in the amount people sleep across the year; however, there has been limited analysis of this seasonal sleep variation using laboratory data. In this retrospective study, 292 patients with sleep disorders who had undergone 3-day polysomnography sleep studies were recruited. Polysomnography is used to diagnose sleep disorders and records blood oxygen levels, heart rate, breathing and brain waves during sleep as well as eye and leg movements. This is performed at sleep disorder units within hospitals or sleep centers.

In the study, patients were advised to sleep with their normal schedule but without setting an alarm clock. The researchers analyzed data from the second night of each study. The team excluded patients taking medication known to influence sleep, people with suspected skipped REM sleep stages and cases of technical failure, leaving a total of 188 patients. The studies were carried out throughout 2019 with a large enough cohort that was spread across the year to enable researchers to calculate month-to-month differences and 90-day moving averages.

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The results showed that there was no statistically significant change in total sleep time between seasons, although patients appeared to sleep for an hour longer in December than in June. However, REM sleep – which has a known link to the circadian rhythm and is impacted by changing light– was affected. For patients in the study, the total time for REM sleep was 30 minutes longer in winter than in spring. In addition, REM latency – the time between falling asleep to the first period of REM sleep – was about 25 minutes shorter in autumn than in spring. This reduction could indicate that less deep sleep occurs in autumn.

The authors concluded that the sleep architecture for patients in the study varied with the seasons. They showed that this was true even in an urban environment where it was previously suggested that seasonal variation in light is limited by higher light pollution and lower levels of natural light.

The team was careful to highlight that further study is required before conclusions are drawn regarding seasonal sleep variations in healthy populations. However, they believe these seasonal differences may be more apparent in those who don’t experience sleep difficulties.

Dieter Kunz (affiliated with both institutions), the study’s corresponding author, added, “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.”