As spring arrives and the clocks move forward, bringing longer daylight hours and warmer temperatures, many people welcome the change. But there is one effect of the seasons that is less obvious — sleep patterns. Research suggests that the human body may need more rest in the winter months than in summer, even for people living in cities where artificial lights dominate the environment.
Seasonal Changes in Sleep
A recent German study, led by Dieter Kunz, head of the Clinic of Sleep and Chronomedicine at St Hedwig Hospital in Berlin, examined 188 urban residents with sleep disturbances. The researchers discovered notable seasonal differences in both REM (rapid eye movement) and deep sleep — the two main stages of rest.
The study found that participants slept an hour longer in December than in June, with REM sleep, the stage linked to dreaming and circadian rhythms, lasting 30 minutes longer in winter. More surprisingly, slow wave sleep (SWS), or deep sleep — which plays a crucial role in repairing tissues, strengthening muscles, supporting immunity, and consolidating memories — was also seasonal. Deep sleep was 30 minutes shorter in September than in February, a finding that had not been reported before.
"These seasonal variations in deep sleep were unexpected," said Kunz. "Deep sleep is usually homeostatic — it depends on how long you’ve been awake — so we didn’t anticipate these changes over the year. More research is needed to understand their functional significance."
The Role of Circadian Rhythms and Evolution
Sleep expert Neil Stanley explains that the seasonal variation in sleep likely reflects evolutionary adaptations. “Humans evolved to follow natural light-dark cycles,” he said. “In winter, when mornings are darker, our brains signal us to conserve energy. In summer, early light encourages waking earlier.”
Even in urban settings with artificial lighting, the study showed that seasonal patterns persist. Exposure to artificial light at night is known to suppress melatonin, the hormone that regulates sleep, but these seasonal trends suggest that natural biological rhythms remain influential.
Health Implications of Seasonal Sleep
The amount and quality of sleep have a direct impact on health. Chronic sleep deprivation has been linked to heart disease, obesity, type 2 diabetes, and depression. Norwegian researchers also found that adults sleeping less than six hours a night had a higher risk of infection than those sleeping seven to eight hours. Those sleeping over nine hours also showed increased infection risk, possibly linked to underlying health conditions.
This highlights the importance of adjusting sleep routines according to seasonal needs rather than maintaining the same schedule year-round.
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Practical Tips for Better Sleep
Kunz and Stanley offer practical advice to help align sleep with seasonal requirements:
- Morning light exposure: Spend 10–15 minutes outdoors in the morning to signal your circadian clock.
- Limit evening light: Avoid bright lights and screen time at least two hours before bedtime.
- Maintain an optimal bedroom temperature: Aim for skin temperatures between 31–35°C (87.8–95°F), which can be challenging during summer heatwaves.
- Children’s routines: Adjust children’s bedtime slightly earlier in winter to accommodate increased sleep needs.
“Most people maintain the same bedtime throughout the year,” Kunz said. “But our research shows that in winter, this can result in losing one to two hours of sleep each night. This could have long-term health consequences if left unaddressed.”
Looking Forward
The study was conducted on participants with sleep disturbances, so further research in healthy populations is needed. However, these findings underscore that our bodies may still follow seasonal rhythms that are often ignored in modern, artificially lit environments.
As the seasons change, adjusting sleep schedules to match our biological needs could improve energy levels, immunity, and overall well-being, ensuring that we remain well-rested throughout the year.
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