Our internal biological clocks, known as circadian rhythms, regulate sleep and wake patterns based on sunlight, and daylight hours vary throughout the year, which can affect the length and quality of our sleep. A group of experts has carried out a study that shows that our REM sleep –the phase in which we dream– lasts longer in winter than in summer and that we have less deep sleep in autumn. Their findings have been published in Frontiers in Neuroscience.
Depending on our chronotype we may be morning larks (who go to bed early and get up early) or night owls (night owls who prefer to stay in bed late in the morning), but regardless our body clocks are determined by the sun. In studies in which people evaluate their own sleep, it has been found that they sleep more in winter, but it is necessary to verify if this is the case with objective measures.
“Possibly one of the most treasured achievements in human evolution is the near-invisibility of seasonality at the behavioral level,” says study corresponding author Dr. Dieter Kunz, based at St. Hedwig from Berlin. “In our study we show that human sleep architecture varies substantially seasonally in an adult population living in an urban environment.”
Adapt our waking time to seasonal changes
Researchers led by Aileen Seidler in Dr. Kunz’s working group at the Charité Medical University Berlin selected 292 patients who had undergone polysomnography (sleep studies) at St. Hedwig Hospital. This type of test is intended for people who have difficulties associated with sleep and is carried out in a special laboratory where patients sleep to assess the quality and type of sleep, as well as its duration.
“Societies need to adjust sleep habits, including duration and time, according to the season, or adjust school and work schedules to seasonal sleep needs”
Patients taking drugs known to affect sleep, technical failures during polysomnography, and REM sleep latency of more than 120 minutes were ruled out, suggesting that the first REM sleep episode had been missed. After these exclusions, 188 patients remained, and although in most cases their diagnoses did not show a seasonal pattern, insomnia was more frequently diagnosed towards the end of the year.
These patients lived in an urban environment with low exposure to natural light and high light pollution, and this should influence light-regulated seasonality, but the researchers found subtle changes across seasons. In winter, the total sleep time was one hour more than in summer, but this data was not statistically significant. However, REM sleep lasted 30 minutes longer in winter than in summer, and it is known that REM sleep is directly related to the circadian clock that is affected by changes in light.
The researchers have acknowledged that a new study involving “a large cohort of healthy subjects” who do not have sleep problems is necessary to validate their results, but suggest that the observed seasonal changes may be even greater in a healthy population. Most of us cannot control waking time because we are forced to adapt to school or work schedules, but society as a whole could benefit from adaptations that allow humans to respond more effectively to seasonal changes. .
Meanwhile, going to bed earlier in the winter could help adjust to human seasonality. “Seasonality is pervasive in any living thing on this planet,” Kunz said. “Although we continue to have flat performance, during the winter human physiology is down regulated, with a feeling of ‘going empty’ in February or March. In general, societies need to adjust sleep habits, including duration and time, according to the season, or adjust school and work schedules to seasonal sleep needs.
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