Our circadian rhythms, which are regulated by the sun, dictate whether we are predisposed to staying up late or waking up early. The changes in day length and light exposure throughout the year could potentially impact the quality and length of our sleep.
But it’s difficult to understand how this works in real life. Even though self-report studies have shown that people sleep longer in the winter, objective measurements are needed to find out how exactly the seasons influence sleep.
In the journal Frontiers in Neuroscience, sleep researchers have recently published data indicating that, even in metropolitan populations with disturbed sleep, individuals have longer REM sleep in winter than in summer and less deep sleep in autumn.
“Possibly one of the most precious achievements in human evolution is an almost invisibility of seasonality on the behavioral level,” remarks corresponding author Dr. Dieter Kunz.
In this new study, they “show that human sleep architecture varies substantially across seasons in an adult population living in an urban environment.”
Scientists led by Ms. Aileen Seidler in Dr. Kunz’s working group at the Charité Medical University of Berlin identified 292 patients at the St. Hedwig Hospital who had had sleep studies called polysomnographies.
Patients who have trouble sleeping are often used for these studies. They are asked to sleep naturally, without an alarm clock, in a special laboratory where the quality and type of sleep, as well as the length of sleep, can be tracked.
Despite the possibility of sleep disorders impacting the outcomes, the study includes a sizable sample group evenly distributed across the year, facilitating the examination of monthly variations.
The team didn’t include patients who were taking medications that were known to affect sleep, who had technical problems during the polysomnography, or whose REM sleep latency was longer than 120 minutes, which implied that the first REM sleep episode had been skipped.
After these exclusions were done, 188 patients were still present. Insomnia was a more prevalent diagnosis towards the end of the year, despite the fact that the majority of their diagnoses lacked a clear seasonal trend.
The patients were located in an urban setting with little exposure to natural light and significant light pollution, which should have an impact on any seasonality controlled by light. Nevertheless, the researchers discovered small but significant variations across the seasons.
It seemed like people slept for around an extra hour in the winter than they did in the summer, but this difference was not statistically significant. On the other hand, REM sleep lasted 30 minutes longer in the winter than it did in the summer.
Light cycles are known to have a significant impact on the circadian clock, which in turn influences REM sleep. Although the authors noted that these findings would need to be confirmed in a group without sleep issues, the seasonal shifts may be even more pronounced in a population that is healthy.
“This study needs to be replicated in a large cohort of healthy subjects,” adds Kunz.
Despite the fact that most people’s waking hours are now mostly determined by their job or school schedules, society may benefit from making adjustments that would enable individuals to adjust to the seasons more successfully. For the time being, trying to adapt human seasonality by going to bed earlier during the winter could be helpful.
“Seasonality is ubiquitous in any living being on this planet,” adds Kunz. “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.”
Source: 10.3389/fnins.2023.1105233
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