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We all have a preferred time for sleeping — a body clock. There are “morning people,” “evening people,” and those in between. Our preferences for when to sleep are called chronotypes. And, increasingly, researchers have been investigating what happens to people whose body clocks are out of sync with the rest of society.
That is: What happens if you’re a late riser living in an early riser’s world?
Scientists have been circling around one answer that’s very concerning: that there are real, and negative, health consequences of being a later chronotype (going to sleep well after midnight and rising later). It may even put you at higher risk of early death.
This past week, researchers at Northwestern and the University of Surrey published a huge study in the journal Chronobiology International of more than 433,000 adults in the UK, who had been tracked for an average of 6.5 years. It found a correlation: Those who reported having a later chronotype (people who are night owls) had a 10 percent increased likelihood of dying compared to people who had an earlier chronotype. And this was true for people of all ages in the study, and for both men and women.
It’s always important to note with studies like these that the 10 percent indicates a relative increase in the risk of death. An individual’s actual risk of dying in any given year is small. Of the 430,000-plus subjects in this study, just a fraction — 10,500, or about 2 percent — died within the study period. These results don’t mean an early death is imminent for late risers.
But it’s still concerning. As the authors note, “any increase” in risk of death “warrants attention.” The analysis also revealed greater rates of cardiovascular disease, diabetes, respiratory disease, gastrointestinal problems, and psychological distress among evening-type people.
It’s hard to know how all these risks interplay with one another, and there’s no clear answer as to why there may be health risks to being a late sleeper.
But here’s a compelling hypothesis: When our biological clock is out of sync with society’s, our whole biology gets thrown off, and many aspects of our lives grow more stressful. Having a very late chronotype is like living in a constant state of jet lag, which takes a toll on the body.
Understanding the science of chronobiology may help us live healthier lives. Or, at the very least, it helps us recognize that some people just like to sleep later than others. And it’s really okay to be this way — we should accommodate and respect it.
Chronotypes are our preferred times to be asleep
Just like it’s pretty rare for a person to be 7 feet tall, it’s pretty rare for some people to not be able to go to bed before 3 am.
Most people — around 50 percent — fall right in the middle of the chronotype bell curve. Average sleep is between the hours of 11 pm and 7 am, give or take an hour.
Below, see the results of the chronotypes of 53,689 Americans charted in a 2017 study in PLOS One. The term “mid sleep” on the x-axis simply means the time people are halfway through their sleep for the night. A mid-sleep of 0 is midnight, a mid-sleep of 4:00 is 4 am, and so on.
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Men tend to vary more on chronotype than women. That just means women are slightly more likely to have an “average” chronotype than men, as you can see in tallest line in the middle of the chart above.
But there are men and women at the extremes on either end. Only around 0.2 percent of adults — one in 500 — have a condition known as delayed sleep phase, which is the chronic inability to go to bed early. People with this condition often have trouble falling asleep before 3 am or even later. The condition is much more common among teens, whose clocks gradually shift earlier as they age.
Some adults are on the other end of the spectrum. About 1 percent of the population has what’s known as advanced sleep phase syndrome. These people prefer to go to sleep around 8 pm.
You can find out chronotype by taking the Morningness-Eveningness Questionnaire (find an online version of it here). Basically, it asks: If you could plan your day however you’d like, what time would you go to sleep and what time would you prefer to wake up? (The researchers in the recent UK used a simplified, one-question version of this test.)
Furthermore, the research finds our internal clocks are influenced by genes and are incredibly difficult to change. If you’re just not a morning person, it’s likely you’ll never be, at least until the effects of aging kick in. As we get older, our clocks nudge us to wake up earlier and earlier.
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People in all chronotypes need around seven or more hours of sleep per night. People with a later chronotype don’t necessarily sleep more hours than those with an earlier one. They just prefer to do it at different times.
To understand why some people naturally sleep later than others, we need to understand the circadian system
The body is an orchestra of organs, each providing an essential function. In this metaphor, the circadian rhythm is the conductor.
The most important thing to know about the circadian system is that it doesn’t just control when we’re sleepy. “Every neurotransmitter, hormone, and chemical in the body cycles with the daily rhythm,” Philip Gehrman, a sleep researcher and clinician at the University of Pennsylvania, said in a 2016 interview. “It’s not just humans; even single-cell organisms follow a circadian rhythm. It really seems to be a fundamental property of life.”
Our bodies run this tight schedule to try to keep up with our actions. Since we usually eat a meal after waking up, we produce the most insulin in the morning. We’re primed to metabolize breakfast before even taking a bite. It’s more efficient that way. For people who are either more morning-oriented or evening-oriented, everything the circadian system controls is delayed and out of sync.
While our bodies keep good time, they’re not perfect. “Our clocks don’t run on exactly a 24-hour cycle,” Gehrman explained. They’re closer to 24.3 hours. So every day, our body clocks need to wind backward by just a little bit to stay on schedule.
For the most part, the sun takes care of this. Exposure to bright light stimulates the brain’s master clock — the suprachiasmatic nucleus — to wind back those three-tenths of an hour.
With night owls, a few things get in the way of this resetting process.
1) Genes: The suprachiasmatic nucleus is the body’s master clock, but it isn’t the only one. Every single cell of the body has clock genes, bits of DNA that flip on and off throughout the day. Like the body as a whole, the cell’s metabolism is scheduled for efficiency. Clock genes regulate the expression of between 5 and 20 percent of all the other genes in the cell.
The action of these genes is believed to feed back into the body’s master clock and help set its time. Scientists have found that small variations in these genes lead to earlier or later rhythms in animals, and are beginning to identify the genes that cause the same effects in humans.
2) It’s also possible that evening-type people have a body clock that runs longer than average. A longer clock means the suprachiasmatic nucleus has to work harder to make an adjustment. When it fails to readjust, sleep times drift later and later into the evening.
3) Later types may be more sensitive to light exposure at night. Bright light at any time of the day tells our bodies it’s time to be awake. This wasn’t a problem back in olden times, when the setting of the sun ended light exposure for the day. In modern times, light from our computers and televisions pushes some evening-type people to stay awake longer.
And, of course, the true answer may be some combination of all three of those reasons — and perhaps some yet to be discovered.
Sleeping less from being out of sync can harm our health
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Scientists have a term for when our body clocks are out of sync with society: social jet lag. Think about how you feel on a Monday morning. After a weekend of sleeping late, you have to wake up hours earlier; it’s like jumping to a new time zone. If you experience that daily, it can put a stress on the body that undermines health.
In a tightly controlled lab study, 24 healthy participants who had their sleep shifted by one hour each day (simulating jet lag) started to look prediabetic after a three-week trial. Their resting metabolic rates dropped 8 percent. “Assuming no changes in activity or food intake,” that “would translate into ~12.5 pounds increase in weight over a single year,” the study, published in Science Translational Medicine in 2012, concluded.
When people experience social jet lag, they’ll often try to make up for the sleep debt on the weekends. But this too is jarring for the body and makes waking up on Monday all the more difficult.
In 2012, researchers in Europe analyzed a self-reported data set of 65,000 Europeans and found “social jet lag significantly increased the probability of belonging to the group of overweight participants.” There’s also correlational research indicating that late chronotypes may be at a greater risk for depression, and that they’re more likely to engage in risky behaviors like smoking.
The hypothesis here isn’t that chronotype inherently causes these negative outcomes, but rather that a mismatched chronotype and daily schedule do.
A 2015 study tracking the sleep of 447 middle-aged adults for a week also picked up on this worrisome pattern. It found social jet lag correlated with insulin resistance — a precursor to diabetes — lower HDL cholesterol (the good kind), higher levels of triglycerides, higher waist circumference, and higher body mass index. These correlations remained even after adjusting for behaviors like exercise, smoking, and alcohol use.
“What I think we’re showing here is that there’s some sort of importance about us ideally being able to work, wake, and match up our schedule as best as we can to what we are biologically suited for,” Patricia Wong, the lab researcher on the 2015 paper, said in a 2015 interview.
If late sleepers want to wake up early, they’re often hit with a double whammy. They’ll be out of sync with society, which stresses the body, but also will have underslept. The research is a bit clearer on this: Short sleep appears to be a significant risk factor for heart disease, metabolic disorders, diabetes, and obesity.
Late sleepers are tired of being discriminated against
In 2016, when I first reported on the science of chronobiology, I spoke to several people with delayed sleep phase, a condition that puts people on the extreme end of the night-owl chronotype. These people have a hard time falling asleep before 2 or 3 am and prefer to sleep until around noon. There’s nothing wrong with their sleep other than that their schedules for it are shifted.
These late sleepers are tired of being judged for a behavior they cannot easily control. If they can’t change their sleep patterns, maybe society should become more accepting of them. We tend to assume that late wakers are the partiers, the deadbeats, the ones who are so irresponsible they can’t keep a basic schedule. The people I spoke to found these assumptions to be personally damaging.
We should follow common sense for a solution. People should be able to sleep when their bodies demand it. Considering the potential health impacts of ignoring our biological clocks, it seems harmless enough to try.
