Sleeping at night in a room with artificial light is known to affect the duration and quality of sleep. Melatonin, the hormone produced in the pineal gland, promotes sleepiness as evening darkness descends, and leads us to wake up as the morning light strengthens. Artificial light during the night hours can disturb sleep patterns by reducing the production of melatonin at an inappropriate time. But what about the fact that the sky becomes light so much earlier in summer than in winter? Does this affect the duration or quality of our sleep?
Although it makes sense that changing day length and light exposure during the different seasons could affect our sleep, this is difficult to measure objectively. Many people wake to the call of an alarm clock in order to get to school or work timeously, and therefore modify their sleep patterns artificially. Some studies where people have assessed the quality and duration of their own sleep have suggested an increase in sleep duration during winter, when day length is shorter, but scientific measures are needed to determine how, exactly, the seasons affect sleep.
Many species of animals hibernate as a way of surviving the harsh conditions of winter, but humans show no such behavioral or physiological adaptation. However, they have been shown to exhibit seasonal patterns in the production of certain hormones and to become less active in winter and consume more calories. A new study of people with sleep problems has now investigated empirically whether the characteristics of sleep vary depending on the time of year.
“Possibly one of the most precious achievements in human evolution is an almost invisibility of seasonality on the behavioral level,” said Dr. Dieter Kunz, corresponding author of the study, based at the Clinic of Sleep & Chronomedicine at the St Hedwig Hospital, Berlin. “In our study we show that human sleep architecture varies substantially across seasons in an adult population living in an urban environment.”
The study was led by Ms Aileen Seidler, a researcher in Dr. Kunz’s working group at the Charité Medical University of Berlin. Data on sleep characteristics were collected from 188 patients (98 women and 90 men, aged between 17 and 81 years) who had undergone polysomnography at the St Hedwig Hospital. These studies are regularly carried out on patients who experience sleep-related difficulties, using a special laboratory 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.
The researchers were interested in variables such as the total duration of sleep, the time spent in REM sleep (the phase during which the brain is more active and dreaming takes place), the time spent in deep, slow-wave sleep, the time taken to fall asleep (sleep latency), and the time taken between sleep onset and the first period of REM sleep (REM latency). They collected data from participants in all months of the year and averaged the results for each month over a full year.
The results of the study, published in the journal Frontiers in Neuroscience, show that, even for an urban population of adults experiencing sleep difficulties, there are clear seasonal effects on certain sleep parameters. Urban dwellers are typically exposed to lower levels of natural light and higher light pollution, but the participants still needed an extra hour of sleep during the winter months than they did in the summer. This result was not statistically significant, although the trend was clearly apparent.
The time spent in REM sleep was an average of 30 minutes longer during the winter months, and this result was statistically significant. REM sleep is known to be directly linked to the circadian clock, which is affected by changing light levels. The researchers also found that REM latency was significantly shorter in winter and early autumn compared to late spring and late autumn. In addition, the duration of deep, slow-wave sleep was largely stable over the year, except for a significant decrease in duration shown in autumn.
The team acknowledged that these results may have been affected by the particular sleep disturbances experienced by the patients, and would need to be validated in a population with normal sleep patterns. However, the fact that a seasonal influence was detected in people with sleep problems may mean that this effect would be even more apparent in a healthy population.
The authors state that this is one of few studies investigating seasonal changes in polysomnography-documented human sleep architecture that looks for subtle changes by comparing months and not just seasons.
The experts suggest that seasonal changes in sleep characteristics may have implications for recommendations regarding sleep routines. For example, people may be encouraged to go to bed earlier in winter in order to fulfil the need for extra sleep time during this season. The twice-annual turning of the clock may also help people to adjust to seasonal changes in physiology that may underlie the sleep changes identified in this study.
“Seasonality is ubiquitous in any living being on this planet,” said 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.”
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