healthy sleep, sleep research, chronobiology, science, insights, Elizabeth Klerman, Harvard, Till Roenneberg, Munich
Anastasiya Vragova/Pexels

Sleep Research: What You Need To Know About Your Body Clock

Leading scientists Elizabeth Klerman and Till Roenneberg explain why sleep is vital to good health and how lifestyle choices can interfere with a good night’s rest…

Sleep is one of nature’s great remaining mysteries. Cats do it, dogs do it, koalas do it (famously, for up to 22 hours a day) and humans do it, of course – but nobody really knows why.

“Despite decades of research and many discoveries about other aspects of sleep, the question of why we sleep has been difficult to answer”, according to Harvard University’s Healthy Sleep page.

As humans, we spend about a third of our lives resting, tuning out of the world around us. At first glance, this may seem pointless, but the science is clear: “Quality sleep – and getting enough of it at the right times – is as essential to survival as food and water”, the National Institutes of Health (NIH) observe.

Timing is essential, as research in the field of chronobiology shows: if we don’t sleep and wake at the appropriate times, our inner clock can become misaligned with our lifestyle – leading to sleep disorders and worse.

In a recent DLD Sync session, we invited two of the world’s most respected chronobiologists to share insights into their research: Elizabeth Klerman (Massachusetts General Hospital, Harvard Medical School) and Till Roenneberg (LMU Munich, Chronsulting) explained in conversation with DLD founder Steffi Czerny why you cannot sleep too much, how school and work hours are messing with our inner clock and what you can do to get a good night’s sleep – which in itself is a fairly new concept.

Find key takeways below, and watch the video of the entire session to learn more.

Think of Sleep Like a Credit Card

When each day seems too short it may be tempting to cut back on sleep. Maybe six hours will do instead of seven? Or five instead of six?

Not a good idea. “Your body needs sleep”, Elizabeth Klerman says, and if it takes several cups of coffee to keep you going through the day, you may not get enough rest at night. “If you feel tired, believe it.”

Trying to short-change your body won’t work, Till Roenneberg warns. “You will be much less efficient after a bad night’s sleep”, he says. “Sleep is important and should be embraced so that we can be optimally awake.”

Think of your sleep budget like a credit card, Roenneberg suggests. “You have a credit for wakefulness. And if you took out too much, you have to pay it back at the end of the day”, just like you need to pay off the debt of a credit card.

Elizabeth Klerman, Harvard University, chronobiology, sleep research

Elizabeth Klerman

Harvard Medical School

“Unlike eating, there’s no way you can sleep too much. Your body will stop you.”

The Right Amount of Sleep

Sleeping in on weekends is fine. Really. While it’s important to keep a regular rhythm, getting enough sleep is even more important.

If you need extra time in bed on Saturday or Sunday “because you’re only getting five hours of sleep during the week, you should sleep in on weekends in order to catch up on your missing sleep”, Elizabeth Klerman advises.

In an experiment, the Harvard researcher gave participants 16 hours of time in bed every day. Initially people slept for around 12 hours per day, then “it gradually went down”, Klerman reports. After a week, younger participants settled at almost 9 hours, older ones at slightly less than 8 hours per day. For the rest of their scheduled bed time they were lying awake.

Conclusion? “There’s absolutely no evidence that you can sleep too much”, Klerman says. “In contrast, there’s a huge amount of evidence that if you get insufficient sleep, it’s bad for every aspect of your body.”

If you do get at least eight hours of sleep each night and still feel tired you may want to consult a specialist, Klerman says, to make sure you’re not suffering from a sleep disorder.

Till Roenneberg, LMU Munich, chronobiology, chrono biology, sleep research

Till Roenneberg


“It’s only because we know that the alarm clock will ring that we get panicky and that we say, ‘We have to sleep!’”

Finding Your Natural Rhythm

Evolution has given humans, like most creatures on earth, a natural rhythm closely attuned to the 24 hours of one day. But modern life keeps messing with our internal clock. Work, school, socializing or binge-watching can interfere with the body’s needs.

When the Industrial Revolution created factory jobs and big cities, traditional sleep habits often got lost. Artificial light extended the day into the night, and employers demanded work shifts at any time of day or night.

“It’s really fascinating how changes by the Industrial Revolution and electricity have changed the way we think of normal sleep, as opposed to what our body used to have”, Elizabeth Klerman says.

In fact, the concept of sleeping through the night is fairly new and “came with capitalism, which wanted us to work for 16 hours at the early stages of industrialization”, as Till Roenneberg points out. “And they gave us eight hours to sleep.”

His own research has taken him to indigenous people in Brazil who still go to sleep shortly after sunset – only to wake up in the middle of the night, chat or eat and then sleep again.

“Nobody told us that sleep has to come in one episode”, Roenneberg says. You may even be predisposed to wake up naturally at night, depending on your chronotype – which determines whether you’re a morning person or a late riser.

“And it’s only because we know that the alarm clock will ring that we get panicky and that we say, ‘We have to sleep!’”, Roenneberg observes. “But this is a cultural phenomenon, not a biological one.”

Social Jetlag

Ideally, all of us could just live in sync with our inner clock. But society typically demands compromises. If you need to take your kids to school in the morning, right before work, how could you  sleep in?

This is what researchers call social jetlag.  “There’s a discrepancy, as if you’re living in the wrong time zone between what your body clock wants, and what your demands are”, Klerman explains.

The scientific concept of social jetlag was originally created by Till Roenneberg, who also coined the term and co-authored a series of scientific papers about it.

If you can’t influence external factors you should at least try to practice good sleep hygiene. “You have to help your body get ready to go to sleep”, Klerman says. That means: try not to have stressful conversations (or horror movies) right before bedtime and keep “approximately regular times of going to sleep, dimming the lights, relaxing and then going to sleep”.

And if possible, work from home. “There are many disadvantages to home office”, Roenneberg says. “But there are also many advantages because you save on the commute time, you can sleep longer.” A recent study showed that many people “actually stop using alarm clocks during home office and lockdown situations”, he reports. “Making work times more flexible will help many people sleep better.”

How Can Technology Help?

If you’re using your smart watch or a dedicated sleep tracker to measure how restful your night was, that’s fine – as long as you don’t put too much faith in technology, Elizabeth Klerman says.

“You shouldn’t use the device to replace what you know about your body”, she advises. Especially since many consumer products are not certified as medical devices.

“There are standard ways of measuring sleep”, the Harvard researcher explains. “And many of these portable devices haven’t been validated. So we don’t actually know whether or not they correlate with any of the health measures that are associated with sleep.”

Better trust your own observations – especially when you’re on the road and you notice that you’re getting tired. “There are a number of studies that show that actually lots of accidents happen within two miles of home”, Klerman says. “In other words, people know they’re tired, and they just want to get home.”

But willing your body into staying awake is extremely risky. “No matter how skilled you are, when the body decides it’s tired, it goes to sleep”, Klerman warns. “People need to recognize that when you need to sleep is not a good time to drive. It might be inconvenient. But it’s better to be alive.”

One day, autonomous cars may solve this issue, of course, driving from A to B while their passengers lean back, relax and sleep. But until then, systems detecting the driver’s level of alertness could save lives, especially if the car intelligently responds to signs of drowsiness, Roenneberg says.

“What the car should do”, he suggests, “is say, ‘I am going to drive you to a parking lot. And I am not going to allow you to drive any further until you have slept deeply for at least 10 minutes.’”

Napping may not replace proper sleep, Roenneberg explains, but it provides a short energy boost, similar to a snack. “A snack is not a meal, and the nap is not sleep”, yet it helps to recover alertness at least for a short while, he says.

In addition, engineers could design cars to enhance a restful  break. The car should give drivers “all the possibilities, both with lying down and with white noise and with no light and so forth, to make you fall asleep”, Roenneberg suggests.

To him, the example shows how important it is that designers and developers talk to scientists, for the benefit of their customers.

“Biology should always be on the minds of technology builders”, Roenneberg argues. “And I can only advise technology to take people who know about sleep and body clocks, and put them into the into the process of developing cars.”

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