If you feel sleepier in the winter and more bright-eyed in the summer, you’re not alone. Your circadian rhythm, which regulates your body clock, is maintained by exposure to light. And in the winter, especially in northern latitudes where the daylight hours are especially short, light can be scarce. While seven to nine hours of sleep is still a healthy benchmark year-round, you may find that you need an hour or two more than you did in the sunny days of summer to feel bright-eyed.
Of course, in modern times, with no shortage of electricity to light the post-sunset (and pre-dawn) hours, we are less limited by the sun than our ancestors might have been. In fact, some scholars believe that before the 19th century, it was common for people to spread a typical eight-hour night of sleep spread out over about 12 hours of darkness, especially during the long nights of winter. They would sleep for about four hours, wake up to pray, meditate, talk, or write by candlelight, and then go back to sleep again until daylight.
These days, it’s easy to flick on a light when the alarm goes off at 6:00am, even if it’s still pitch dark outside in January. But as many as 90 percent of people’s moods and energy levels are affected by changes in the seasons. For about four to six percent of individuals, that can escalate to seasonal affective disorder (SAD), a form of clinical depression that fluctuates based on the time of year. Meanwhile, as many as 20 percent experience a more mild form of winter blues. And with both of these diagnoses can come symptoms that affect your sleep, including loss of energy and needing 1.75 to 2.5 extra hours of sleep each night (for SAD and the winter blues respectively).
Sleeping too much can be unhealthy, regardless of the season, so if you’re already getting seven to nine hours of zzz’s per night and you’re still not feeling rested, talk to your doctor about getting tested for a sleep disorder. If it’s winter—and lack of light that has you down—simple lifestyle tweaks like getting regular exercise (ideally in sunlight) or using an artificial light box may help to get your body clock back on track.