why do we feel sleepy in winter?
As the clocks change and the days lengthen, new research suggests we should think about what this means for our bedtimes.
After a long, hard winter, the arrival of spring is often a welcome change. The Sun stays up longer, the days become warmer, the first flowers bloom, and clocks in many countries advance into daylight savings time to lengthen our evenings.
But there is one change that is likely to be overlooked as we inch closer to summer: you begin to sleep less.
Many of us are familiar with the struggle to get out of bed in the morning during the winter, preferring to hit the snooze button instead. This, according to scientists, is not surprising.
According to new research, humans may require more sleep during the dark winter months than they do during the summer. This need appears to exist even in city dwellers, where artificial lights are expected to interfere with the natural influence of daylight on our sleeping patterns.
“Our study shows that even in an urban environment with only artificial light, humans [experience] seasonal sleep,” says Dieter Kunz, one of the study’s lead authors and head of the sleep and chronomedicine clinic at Berlin’s St Hedwig Hospital. “I would expect seasonal variations to be much higher if the patients were living outside and only exposed to natural light,” he adds.
Previous research has found that exposure to artificial light before bedtime can suppress melatonin secretion, the hormone produced by the pineal gland that regulates our circadian clock, the natural sleep-wake cycle that repeats every 24 hours and makes us feel sleepy.
However, the German study, which used detailed sleep recordings of 188 patients who lived in cities and had trouble sleeping, discovered that even when exposed primarily to artificial lights, the participants experienced seasonal variations in REM sleep, which is directly related to our circadian rhythm. In fact, the participants slept an extra hour in December than they did in June. Their REM sleep, which is the most active stage of sleep when we dream and our heart rate rises, was 30 minutes longer in the winter than in the summer.
“The fact that it goes in parallel with seasonality makes sense,” says Kunz, because REM sleep is regulated by the circadian clock.
The German study, on the other hand, discovered that even when exposed primarily to artificial lights, the participants experienced seasonal variations in REM sleep, which is directly related to our circadian rhythm. In fact, participants slept an extra hour in December than in June. Their REM sleep, the most active stage of sleep during which we dream and our heart rate increases, was 30 minutes longer in the winter than in the summer.
“The fact that it coincides with seasonality makes sense,” Kunz says because REM sleep is controlled by the circadian clock.
“We did not expect the seasonality of deep sleep,” says Kunz. “Because deep sleep is a homeostatic process, it is not governed by the circadian timing system. This means that the longer you are awake, the more deep sleep you will require to replenish your energy.”
More research, according to Kunz, is needed to understand why we require less deep sleep in autumn than in winter. “We still have no idea what it means in terms of functionality.”
It is also worth noting that the study was conducted on patients with sleep disorders such as insomnia, so it will need to be repeated in a healthy population to confirm that these effects are seen more widely.
According to Neil Stanley, a sleep expert at Sleep Station, an online provider of cognitive behavioral therapy for insomnia, it’s “puzzling” that we don’t require more deep sleep during the winter.
“Slow wave sleep is thought to be the most important [sleep phase] because it is involved in memory, learning, and immune system optimization,” Stanley says. “According to popular belief, we would prioritize [deep sleep] over everything else. If you go an entire night without sleeping, you will make up all of your lost deep sleep but only half of your lost REM sleep the next night “he claims.
Exactly why our sleep changes with the seasons could be hidden in our evolutionary past, says Stanley. “We have evolved to dark-light cycles, so when we wake up on a winter’s morning and it’s dark, our brain is going ‘I can’t do anything…there’s no point leaping out of bed’.”
A good night’s sleep, however, is also temperature dependent. Our bodies require a skin temperature of 31-35C (87.8-95F), which we can easily control in the modern world with central heating. This can be more difficult to control in the summer, when higher temperatures are more common, especially during heat waves. (How to stay cool and sleep during a heat wave.)
Should we consider changing our sleeping habits throughout the year if the findings of Kunz’s study are correct?
According to Kunz, most people have a consistent sleep pattern throughout the year. They go to bed around 10.30-11 pm after watching TV and wake up around 7 am to go to work. According to Kunz and his colleagues’ study, children are especially encouraged to stick to a consistent bedtime routine. Because school and work schedules dictate when we need to get up in the morning, it may be worth going to bed a little earlier during the winter to account for the “increased sleep need,” they suggest.
When we know we need more sleep in the winter, it doesn’t make sense to stick to the schedule we make when we’re at our best in the summer,” Kunz says. “Our study shows they will be missing out on one to two hours of sleep each night during the winter if they stick to the same sleep patterns all year,” Kunz says. “I doubt this will help our health.”
According to the Centers for Disease Control and Prevention, a lack of sleep increases the risk of developing type 2 diabetes, heart disease, obesity, and depression.
A recent Norwegian study found that patients who visited their doctor and reported sleeping less than six hours per night had a higher risk of infection than those who slept seven to eight hours. Patients who had chronic sleep problems were more likely to require antibiotics. The study discovered that people who slept for more than nine hours per night were at a higher risk of infection.
“A likely explanation for these findings could be that having an infection causes disturbed sleep or increased sleepiness, or that having an underlying disease increases both sleep and infection risk,” says lead author Ingeborg Forthun of the University of Bergen in Norway.
“Because infections are more common in the winter, it may be necessary to sleep more [then] to help ward off infection,” Forthun says.
At the very least, it may make you feel less sluggish when your alarm goes off on a cold winter morning.
HOW TO IMPROVE YOUR SLEEP
“Get as much natural light as you can in the morning hours so that your circadian system knows the day has begun,” Kunz advises. “It’s critical that kids spend at least 10-15 minutes outside before going to school, [seeing] the sky,” he adds.
People can tolerate varying levels of light at night, according to Stanley, but he advises avoiding bright lights and phone screens two hours before bedtime.
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