On November 6 at 2am, we’ll be setting the clocks back an hour. While this small shift leave us with a blissful extra hour of sleep – and who can complain about that – the shorter days and longer periods of darkness (grumble) can leave some people feeling blue and groggy. Even turning the clock back just 60 minutes – or ahead in the spring – can mess with our circadian rhythm, the 24-hour cycle our body operates on. In addition to regulating how tired or awake we feel during the day, this internal biological clock coordinates hundreds of cellular events that happen in the body, like the release of cortisol and the rise and fall of body temperature or blood pressure.
“If you think of all the molecular, cellular, and physiological processes in your body as functioning as an orchestra, then the master biological circadian clock serves as the conductor to make sure processes such as sleep, hormone secretion, metabolism, body temperature, and immune function occur at the correct time of day,” says Randy J Nelson, PhD, professor and chair of the department of neuroscience at the Ohio State University Wexner Medical Center.
The circadian rhythm is a pretty amazing process that happens every day and has been chronicled for thousands of years – the first accounts date back to the fourth century BC. Ahead, read 10 more facts about this ancient internal clock: what keeps it going, what slows it down, and what it’s all about.
Yes, even an hour can throw you off
Ever seem like the time change leaves you in full-on zombie mode the next day? You’re not imagining the sluggishness. The most our bodies can adjust is an hour a day, but sometimes even that hour is too much to handle, says Phyllis Zee, MD, PhD, director of the Sleep Disorders Center at Northwestern Memorial Hospital. Studies have found that there’s an increase in heart attacks and traffic accidents on the Monday after the time hop.
…which explains why jet lag messes with us
If a mere 60 minutes is enough to wreak havoc on humanity, you can only imagine how travelling several hours across time zones impact the body, physically and mentally. “Jet lag happens because your body is happily running on its own clock, but suddenly has to operate on a new one and often very quickly. You might want to sleep in the middle of the afternoon or be wide awake into the night because that has been your rhythm,” explains Zee. Prevent jet lag by gradually adjusting your sleep schedule before a trip, resisting the temptation to nap during the day, and spending plenty of time outdoors to help make the transition a bit smoother.
People can have different clocks
Most people operate within a similar time frame: Wake up in the morning and fall asleep in the evening (within a range, of course), which explains why shift work is so troubling, since it goes against the natural rhythm of the body. But there are variations, like the larks who like to wake up early and the owls that prefer to stay up later and sleep in. Then there are also differences in age. “There is a natural shift in most people during the teenage years when they tend to move toward more of an evening type, which is more prominent in boys versus girls,” says Colleen McClung, PhD, associate professor of psychiatry at the University of Pittsburgh School of Medicine. “Then as people get older, their rhythms start to shift the other way such that elderly people wake up early in the morning and go to sleep earlier in the evening,” McClung adds.
Women are more likely to be larks
Scientists think there might be differences between men and women, too. A 2013 study from Brigham and Women’s Hospital found that women’s clocks were 6 minutes faster than men’s, meaning they were more likely to call themselves early birds. While, again, 1/10 of an hour doesn’t sound like much, researchers speculate it could mean women aren’t getting quality sleep since instead of turning in early, they’re doing housework or helping with homework in the evening.
Light makes a huge difference.
Bright light — whether it is natural sunshine or the glow emitted from a smartphone — may not seem like a big deal, but it can set off a whole cascade of reactions. “The brain provides central control of circadian rhythms,” says George Brainard, PhD, professor of neurology and director of the Light Research Program at Thomas Jefferson University. “As soon as light enters the eye, it stimulates the retina, and signals are sent to the hypothalamus. In the hypothalamus, there are two small nuclei that are a major part of our inner clock. They send information about light, darkness, and biological time throughout the nervous system and the entire body. This process allows us to be alert and active during the daytime and fall asleep in the night,” Brainard says.
That’s why going to sleep with your smartphone will keep you up longer. “The receptors in your eyes are most sensitive to short wavelength light which is emitted by the sun in the morning but also by cell phones and computer screens,” says Nelson. “This can trick your circadian clock into thinking it’s still time to stay awake and throw off the timing of internal physiology, including sleep, mood, and metabolism,” adds Nelson.
Exercise could impact your rhythms — to an extent
There has been some buzz — based on a 2012 study of mice by researchers at the University of California — that there is an “optimal” time to hit the gym. Researchers found that mice that used a running wheel later in the day had sturdier internal clocks than those who did so in the morning. Does it mean you should swap your AM spin class for a later one? Brainard doesn’t think so — yet. “Some exercise is better than no exercise,” he says, “but we all advise against working out right before bedtime. Physical activity hikes up responses like heart rate and cortisol levels, which make it harder to fall asleep.” If you’re not a daytime gym goer, aim to work out no later than 3 hours before bed.
You CAN have a low-grade version of SAD
Seasonal affective disorder (aka the winter sads) tends to show up right when the days start getting shorter, and scientists think that the drop in sunlight could be to blame. Sunshine is thought to hike up serotonin — one of the chemicals that boosts mood — in the brain. “People with SAD, a diagnosable, psychiatric condition, see profound changes: They want to sleep 12 to 14 hours a day, have intense carbohydrate cravings causing significant weight gain, and experience depressed mood,” explains Brainard. “But 20 to 25% of people experience a subclinical version of SAD, meaning they may feel more tired, less happy, have modest weight gains, and want to sleep in an hour or two more.” Light therapy has been shown to be an effective intervention for both, and one study showed that a morning outdoor walk in sunlight was effective in treating SAD.
Blame your rhythms for that afternoon slump
Every day between 2 and 4pm, the majority of the human race enters a lull, complete with sudden grogginess and a lack of focus. What’s happening? “Your circadian rhythm begins to dip about 8 hours after you get up,” says Zee. “You enter a cycle of sleepiness that could explain the inability to work and be productive because your body is ready to rest,” Zee adds. Some experts advocate taking a short power nap to perk up — no more than 30 minutes or else you risk not being able to fall asleep at night—but if you’re office-bound and without a space or ability to get some mid-afternoon shut-eye, getting up for a walk can help zap lethargy, too.
Follow your clock to boost immunity
A 2016 study of 276 people found that flu shots given in the morning were more effective than those done in the afternoon. Participants were randomly jabbed either between 9 and 11am or 3 and 5pm. Those who received their flu shot early produced significantly more antibodies. “The immune response has a circadian rhythm such that it is primed to respond the strongest in the morning,” says McClung. “This is likely when humans encountered the most environmental threats that could make us sick, so through evolution the immune system has responded accordingly,” McClung.