Biologist Till Roenneberg in an interview

Mr. Roenneberg, we keep hearing that we have an internal clock and it almost sounds like a cliché. What is actually behind it?

Many use this as a phrase, but there is more to it than that. So much so that the Nobel Prize in Medicine was awarded five years ago for research into the internal clock. So it’s real biology. To understand the internal clock, one should understand what we have clocks for. Why do you have a watch?

To tell the time, to structure the day and to get to appointments on time.

Exactly. You want to structure your own day in coordination with others. We have four cycles on earth that structure our environment. These are the tides, the day, the lunar month and the year. We cannot predict which lottery numbers will be drawn on Saturday. However, it is very easy to predict when the Elbe will reach its lowest point in Hamburg. This is the difference between linear and cyclic time. In this respect, the internal clocks have exactly the same value as other clocks. We want to structure something and prepare processes for the future.

Till Roenneberg is a chronobiologist and professor emeritus at the Ludwig Maximilian University in Munich. He is also President of the World Federation of Chronobiological Societies and visiting professor at the British University of Oxford, among others.

How was it discovered that nature and we are driven by internal clocks?

In 1726, a French astronomer placed mimosa plants that open and close their leaves in the dark. He found out that the mimosas still open and close their leaves every day, even in the permanent darkness. So it couldn’t just be passively due to the light-dark change. In the 20th century researchers found this in animals as well. They checked whether a rhythm, such as being awake and sleeping, is dependent on a light-dark cycle or whether it continues when other conditions remain constant – and it continued.

How does this show up in our body?

Every single cell in our body has an internal clock, and it has to actively adapt to its rhythmic environment. This rhythmic environment is passed on by an organ in the brain that only has the task of looking out through the eyes and telling all cells: “Now it’s light” or “Now it’s dark”. This is then translated into a rhythm in the blood and nervous system.

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Light and darkness have become weaker as timers.

What consequences does the internal clock have for our daily life?

There are different chronotypes, so-called larks and owls. The larks tend to get tired and get up earlier. The owls can stay up late and get up later. In between there are the pigeons. I introduced the term because the majority of people lie between these extremes. But this distribution between the types is very wide today. That means the owls and the larks are getting farther and farther apart.


Back then, people were exposed to a lot of light during the day because they were often outside. At night it was completely dark again because there was no electric light. Light and darkness are the most important timers for the internal clock in the brain. But now we mostly live indoors and get a thousand times less light than, say, a farm worker. And at night, in turn, we’re exposed to a lot more light than we used to because we turn on the lights. Therefore, light and dark as timers have become weaker. At that time, the earliest larks and the latest owls were about two and a half hours apart. In today’s industrial society it is twelve hours. Before industrialization, if you woke up on your own at 10 a.m. and couldn’t fall asleep until 2 a.m., then you were definitely suffering from some kind of illness. But nowadays this is quite normal.

So the structured everyday work and man-made influences in general can disturb our internal clocks?

Exactly. The internal clock itself only has a positive effect on our daily life because it takes care of our health. But humanity is trampling on the biology of the individual, our internal clocks. And we do that on many levels. Not only do we work indoors and turn on the lights at night, but we also keep school or work starting at 8 or 9 a.m. at the same time. This is a problem for children and young people in particular: up to the age of 20, the internal clock of children and young people is getting later and later. Only then does it get progressively earlier over the years. From this we can see that it is actually nonsense to leave 14 to 20-year-olds sitting at school early in the morning. That’s pretty much the middle of the night for them.

How else do we disturb our inner clock?

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On the one hand with the so-called summer time. Because of her, we have to get up an hour earlier, adding to the already severe social jetlag. Social jetlag measures how distant my internal clock has to live from the external clock.

Coffee also has an influence. We now know that coffee can definitely set the internal clock. But only if you drink very little coffee. That’s the trick. If you always drink a lot of coffee, you need a lot of it to even wake up.

When we need an alarm clock, we haven’t finished sleeping.

What is the best way to adapt to your own internal clock?

We can best adapt to it by living by it. We still have the same mechanisms and genes as before industrialization. A colleague of mine from Colorado demonstrated this while camping with college students. He measured her chronotype before and after. The result: The distribution of the different chronotypes was much narrower when camping and the students generally got up earlier. In addition to artificial light, there is also a device that 85 percent of the population uses almost every day, with which we trample our internal clock almost every morning.

The alarm clock?

Exactly. If we need it, we haven’t slept through yet. The fact that we almost all need an alarm clock on working days shows us that our inner clocks are too late to adapt to the given social rhythm. That is why we must interrupt our sleep, one of the most important guarantees of health. And that has consequences. So the answer to the question: “How can you live according to your own internal clock?” is: By throwing away the alarm clock. Society needs to rethink this. Because we could create a lot less disease by being mindful of the chronotypes.

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Employers should offer employees the option of waking up without an alarm clock and then coming to work.

And what could that look like?

We tested this at the manufacturer Thyssenkrupp. We chronotyped a workforce that worked in shifts. The owls then no longer have to do morning shifts and the larks no longer have to do night shifts. The result: they all slept an extra hour a day. So embracing individuality would have huge health benefits. Employers should therefore offer employees, if at all possible, to wake up without an alarm clock and then come to work. After all, you don’t want to pay for the first two hours of inefficiency with coffee breaks, but use the employees’ best time and reduce sick leave. Everyone benefits from this situation, employees and employers.

Working against the inner clock increases health risks. Which are they specifically?

Very easily. Ask yourself what most of your family died from. If you now find that most of you died of cardiovascular problems or became demented, then you can predict what is very likely to die of you too.

I guess it was actually cancer in my family.

Then there is a high probability that you could also get cancer at some point. Imagine a graph with age on the horizontal axis and probability of dying on the vertical. From about five to 50 years of age, the probability remains very low, but then increases like a ski tip. Life against the internal clock lassoes the end of this line and pulls it into the earlier years. This increases the likelihood that by the time you are 80 you will get a disease that you would probably get by the time you were 90. This is always coupled with not getting enough sleep at the right time. The job of sleep is to ensure that while you are awake, you do not make mistakes and can function optimally.

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