We know that as the days get shorter, we tend to sleep longer - and that the tendency to sleep longer in autumn and winter is only exacerbated for those of us with seasonal affective disorder (SAD). However, there are some sleep benefits we experience when leaving the summer months behind such as the return to standard time, a decrease in air pollution and lower temperatures, all of which can help our bodies fall asleep.
Seasonality and Sleep Duration
A recent study in Japan confirmed an increase in sleep duration in participants by an average of 19 minutes in the winter months compared to the summer months. However, the researchers noted a large variation dependent on age, gender and residential area. Younger people (ages 15-39) were more likely to experience difficulty maintaining sleep and excessive daytime sleepiness (EDS); people in their middle ages (ages 40-64) were more likely to experience difficulty initiating sleep and early morning awakening; while the older population (ages 65-89) observed no such changes. The study's results suggest that seasonality may affect sleep duration, but it may not affect everyone in the same way.
Seasonal Affective Disorder
SAD is a recurring depressive disorder with a temporal pattern occurring in the fall and winter and remitting in the spring. Both hypersomnolence and insomnia can be exacerbated in the darker months for those who experience SAD. Scientists believe that seasonal light variation contributes to the shift in our circadian rhythm. The fall and winter onset of SAD can cause food cravings, oversleeping, tiredness and weight gain. General SAD symptoms include irritability, hypersomnolence, delayed sleep time, anxiety, and withdrawal. The Seasonal Pattern Assessment Questionnaire (SPAQ) is the most widely used questionnaire to evaluate a patient's mood and behavior in regards to seasonality. Including an assessment of a patient's sleep during the summer months could indicate if hypersomnolence and delayed sleep timing are seasonal in nature.
Decreased sunlight can cause a drop in serotonin levels and seasonal changes also interfere with the body's melatonin levels, impacting mood and sleep patterns. Risk factors for SAD include age, gender and family history. As many as 5% of high school students have already developed SAD, which is about the same for adults. Populations over age 30 are less likely to be affected if they weren't affected before age 30. Additionally, three out of four cases of SAD are women. Because the onset of SAD is linked to a lack of sunlight, populations that work night shift, as many sleep technologists do, are also at greater risk.
Perks to Cold and Darkness
While SAD is more prevalent during the fall and winter in the northern hemisphere, there are some sleep disruption factors that we leave behind during the cooler, darker months. The Sleep Heart Health Study found that an increase in air pollution levels during the summer months may be partially responsible for the increase in sleep disordered breathing (SBD) during the warmer seasons.
Respite From Daylight Saving Time
We are soon approaching the end of the controversial daylight saving time (DST), which ends at 2 a.m. on Sunday, Nov. 7, 2021. The rest of the year, November through early March, is known as standard time. Several studies have pointed to an increase in heart attacks and strokes during the first few days after DST begins in March. One study showed a 21% decrease in heart attacks and strokes the Tuesday following the fall return to standard time.
The Effects of Temperature on Sleep
Ambient temperature is one of the most important factors in the quality of our sleep and we may benefit from the decrease in temperature during the autumn and winter months. Wakefulness increases while slow wave sleep (SWS) and rapid eye movement (REM) sleep decrease when our bodies are exposed to heat. As we prepare for sleep, blood vessels in our skin dilate to induce heat loss - and being exposed to elevated ambient temperatures can prevent the body from shedding heat, increasing our risk of poor sleep. Our body's thermal load is heightened when heat is paired with humidity. Greater cold exposure chiefly affects REM sleep during which the response to ambient temperature is blunted, therefore the ideal temperature for sleep is between 60 to 67 degrees Fahrenheit.
Tips and Takeaways for Winter Sleep Hygiene
General tips for improving sleep during the fall and winter include:
- Exposing the body to natural light, especially in the morning, or using light therapy
- Lowering the thermostat temperature before bed
- Taking a warm bath two hours before sleep
- Minimizing alcohol consumption and smoking
Light and temperature are two of the greatest contributing external factors to our circadian rhythm and our sleep quality. Seasonality affects how we sleep, but it doesn't appear to affect us all in the same way. It is important to take into account factors such as age, gender and family history when assessing the role of seasonality in sleep disturbances.