A study has shed light on why humans may need more sleep during the winter.
The analysis of people in sleep studies found that people get more REM, or rapid eye movement, sleep in the winter.
If the findings can be replicated in people with healthy sleeping, this would provide the first evidence for a need to adjust sleeping habits to the season, perhaps by going to bed earlier in the darker and colder months, researchers say.
During REM sleep, brain activity increases and people may dream.
Normal sleep starts with three stages of non-REM sleep at first, followed by a short period of REM sleep.
Whether people consider themselves morning people or night owls, our body clocks are set by the Sun.
Therefore, in theory, changing day length and light exposure over the course of the year could affect the amount and quality of sleep.
Findings from researchers studying sleep difficulties suggest that even in an urban population experiencing disrupted sleep, humans have longer REM sleep in winter than summer and less deep sleep in autumn.
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“Possibly one of the most precious achievements in human evolution is an almost invisibility of seasonality on the behavioural level," said Dr Dieter Kunz, corresponding author of the study, based at the Clinic of Sleep and Chronomedicine at the St Hedwig Hospital, Germany.
“In our study we show that human sleep architecture varies substantially across seasons in an adult population living in an urban environment.”
Although the people involved in the study were based in an urban environment with low natural light exposure and high light pollution, which should affect light indicating the season, the scientists say they found subtle but striking changes.
While total sleep time appeared to be about an hour longer in the winter than the summer, this result was not considered statistically significant.
But REM sleep, which is known to be directly linked to the circadian clock that is affected by changing light, was 30 minutes longer in the winter than in summer.
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While the researchers acknowledge that these results would need to be validated in people with no sleep difficulties, the seasonal changes may be even greater in a healthy population.
While most people’s waking time is largely out of their control, due to school or work schedules, society might benefit from changes that would allow humans to respond more effectively to the different seasons, the scientists say.
In the meantime, going to sleep earlier in the winter might help to accommodate human seasonality.
“Seasonality is ubiquitous in any living being on this planet," Dr Kunz said.
“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.”
A team of scientists led by Aileen Seidler in Dr Kunz’s working group at the Charite Medical University of Berlin, recruited 292 patients who had undergone sleep studies called polysomnographies at the St Hedwig Hospital.
These studies are regularly carried out on patients who experience sleep-related difficulties.
A special laboratory is used where patients are asked to sleep naturally without an alarm clock, and the quality and type of sleep can be monitored, as well as the length of sleep.
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The researchers say that although the sleep disorders could affect the results, this makes for a large study group evenly spread throughout the year, allowing for the investigation of month-to-month differences.
After exclusions were made for people taking sleep-affecting medication, technical errors and for those who may have skipped the first REM stage, 188 patients remained.
Most of their diagnoses showed no seasonal pattern, but insomnia was more commonly diagnosed towards the end of the year.
The findings are published in the Frontiers in Neuroscience journal.
