Human sleep patterns appear to change with the seasons

The changing seasons have an impact on various forms of life on Earth, including deciduous trees shedding their leaves in the fall and animals hibernating during the winter months. A recent study by Aileen Seidler and colleagues (2023) published in the journal Frontiers in Neuroscience has found that humans also experience marked changes in the duration and structure of their sleep throughout the year. Scientists from the Clinic for Sleep & Chronomedicine at St. Hedwig Hospital in Berlin conducted the research, which involved 188 volunteers spending three nights in a laboratory setting to closely monitor their rest.

Using a process called polysomnography, the volunteers' brain waves, blood oxygen level, heart rate, breathing, eye movements, and leg twitches were all recorded. The data revealed that subjects tended to sleep longer during the winter, by up to 60 minutes, and took about 25 minutes shorter to enter into rapid eye movement (REM) sleep during autumn than spring. They also experienced about 30 minutes more REM sleep on average in winter than spring and underwent a swift and sudden drop in slow-wave sleep during the fall months.

The findings are in line with a previous study conducted by the authors in which thousands of subjects subjectively reported sleeping longer in the winter. However, the study group was relatively small, the subjects all had a condition that prompted difficulty sleeping, and the difference in sleep duration between summer and winter was statistically non-significant. The authors cautioned that the study needs to be replicated in a larger cohort of healthy subjects.

Despite the modern world's mastery of artificial light, which was thought to have mostly rid humans of the tyranny of sunlight, the study found that sleep still showed seasonality. The authors suggest that improvements can be made by accounting for the increased sleep need in winter and going to bed earlier. They note that societies need to adjust sleep habits, including length and timing, to the season, or adjust school and working schedules to seasonal sleep needs. In short, even though humans have mostly made themselves masters of the planet, they do not yet appear to have overcome their ingrained seasonality.