IF YOU FIND YOURSELF feeling less alert when the weather turns cold and wondering what happened to all that boundless summer energy, you’re not alone.

About six percent of Americans experience Seasonal Affective Disorder (SAD), and another 14 percent regularly feel the milder “winter blues” as their energy levels drop in the fall and winter. In fact, people around the word who live far to the north – think Norway and Sweden – experience significant changes in their sleep patterns as the seasons change.

For Scandinavians, Canadians and lots of folks living in northern climes, sleeping in the winter is quite different from sleeping in the summer – changes that don’t affect populations living between the Tropic of Cancer and the Tropic of Capricorn.

What Causes Changes in Seasonal Sleep Patterns?

Seasonal sleep patterns are influenced more by daylight than temperature. Sure, you may want to curl up in a blanket for a long night on the couch when it’s cold out, but that’s not really what’s making you feel so lethargic in the winter.

As the days start getting shorter than the nights in late September, the gradually decreasing amount of daylight means that your body is being signaled to sleep for longer periods.

This trigger is due to a combination of your circadian rhythms and your biological clock. You body’s master clock mechanism is located very close to your optic nerve, which in turn is triggered by daylight. This is why your body wakes up when the sun comes up in the morning, and why you may find it difficult to nap on a bright day.

Your body responds to the short-term patterns of night and day, but the length of each changes throughout the year, triggering different levels of sleepiness in different seasons.

Sleeping in Winter vs. Sleeping in Summer

Most people who live in northern areas feel more tired and want to sleep for longer periods in the winter. Because the sun sets much earlier in November, December and January, you’ll probably feel ready to turn in well before the 10 o’clock news comes on.

It also may feel harder to wake up, especially if your alarm clock is set to go off before the sun rises. To stay on schedule, you’re asking your body to fight its own internal clock, which can lead to feeling sluggish, groggy and irritable.

In the summer, on the other hand, you may feel more energetic overall, thanks to having a much longer stretch of daylight and a more natural wake-up call as the slowly mounting dawn helps awaken you more naturally.

Interestingly, researchers have found that the extra daylight often encourages people to go to bed earlier and wake up earlier than they do in the winter months. The upshot is that you probably feel well-rested and more energetic in the summer, even though you’re not actually getting any extra sleep.

Modern Solutions to an Old Problem

While Medieval monks simply rang their church bells earlier or later as the amount of sunlight waxed and waned, today you can harness the power of electricity to give yourself some extra light and adjust your circadian rhythms appropriately with light box therapy. After all, spending some time in artificial sunlight is better than huddling under your blanket in the darkness, so it’s worth a try to improve your mood and help you sleep better at night.