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Chronotypes and social jetlag: a health issue

Interview with Till Roenneberg, a chronobiologist at LMU Munich

Published: 3 Sep 2020
Increasing segments of the population are beginning to understand the importance to their health of a regular sleep-wake pattern, but for many it is still difficult to put this knowledge to good use. Always managing to fall asleep eight hours before the point when one needs to wake up, or alternatively, only sleeping for a few hours without suffering the day after, seems to be a mixture of natural talent and iron self-discipline: in reality, the effect of one’s own “chronotype” and light plays a role in influencing this.
Chronotypes and social jetlag: a health issue

Till Roenneberg’s books have been translated into numerous languages

We know that the light conditions in the modern world, also due to our tendency to stare at backlit screens until late, influence the production of melatonin in our body, causing an offset in our sleep-wake pattern and increasing the risk of diabetes, obesity, cardiovascular illness, cancer and depression. We know that lighting designers and architects are starting to design buildings and lighting systems that don’t distort but rather accompany our biological rhythms.

That said, the biological rhythm is not the same for everyone: so research still needs to be conducted into the relationship between the individual Circadian clock and the times imposed by society.

To find out more about this subject, we interviewed Professor Till Roenneberg, chronobiologist at the Institute of Medical Psychology at the LMU Munich, Bavaria, propagator and author of books like Internal Time: Chronotypes, Social JetLag, and Why You’re So Tired (Dedalo, 2015).

What is a chronotype? Is it genetic or do external factors play a role? 

Chronotypes are an expression of the circadian clock, a fundamental biological “program” found in all organisms down to cyanobacteria. Now, fundamental biological programs almost never depend solely on genetics or environmental factors. For example, think of body size. The genetic component is strong: if your mother and father are very tall, it is unlikely that you will become very small; not impossible, but unlikely. However, if you grew up during the war and were malnourished as a child, you will not become very tall. Yet again, under the same conditions you will be taller than others whose parents were smaller than yours. Much in the same way, chronotype is the result of both genetics and environment.

How are chronotypes classified?

There are not just two – morning types and evening types – but a continuum. Just like you have very few people who are either very tall or very small, and most body heights in the middle, you also have very early chronotypes – extreme larks – and very late types – extreme owls – but a whole continuum between them.

Does the individual chronotype achieve stability or does it keep changing?

The word “-type” in “chronotype” is unfortunate because it seems to refer to something static in nature, like having blue eyes or black hair or black skin as an adult. There is a fixed genetic component, but chronotype does change very much based on environmental factors. The circadian clock is synchronised with the light-dark cycle, so a change in your exposure to light will translate into a change in your chronotype. The biological clocks of a given group of people will be later if they live in a city than they will be if they live in the countryside.

Are there chronotypes that inherently involve a sleep disorder?

I live in an old farmhouse built in 1800. The doors are extremely low, because people were very small at the time. Now, if you are tall you might injure yourself by banging your head on the frame, but that does not mean that you’re sick: it just means that the interaction between your height and those doors will give rise to problems. The same happens with chronotypes. If people can live within their own biological time zones (according to their chronotype) they will have no sleep problems, they will not have an increased chance to develop depression or other diseases. If, on the other hand, they are forced to live outside of their biological time zones (against their chronotype), there will be a friction, predominantly starting to manifest with sleep disruption and leading on to increased risks of developing most types of cancer, metabolic or heart diseases. We have called “social jetlag” this friction between how you have to live by societal times and how your body wants to live. It is a quantification of the misalignment between body clock and social clock, and it is the reason why people are more likely to develop illnesses.

When you say “quantification”, do you actually mean that social jetlag can be measured?

Yes, it can be measured as the difference between your sleep timing during the work week and that during the weekend. The more hours and minutes of social jetlag you suffer from, the higher your probability of having a sleep disorder or any other related health issue. In order to calculate social jetlag, we introduced the concept of the “midsleep point”, the time of night, when you are halfway through your sleep. So, if you sleep from midnight to 8am, your midsleep point will be at 4am; if you sleep from 10pm to 6am, you will have slept the same number of hours but your midsleep point will be at 2am. Social jetlag equates to the difference between the midsleep point you show on weekends and the midsleep point you show on weekdays.

A lecture given by Roenneberg on social jetlag at DLDwomen 2013

We have called “social jetlag” this friction between how you have to live by societal times and how your body wants to live. It is a quantification of the misalignment between body clock and social clock, and it is the reason why people are more likely to develop illnesses.
In order to reduce social jetlag, then, we need to adjust the timing of our sleep in the workweek to the timing of our sleep in the weekends?

Lovely! You’re obviously from Europe, because you immediately recognised that what has to be changed is the timing during the workweek. Most of the Anglo-Saxon world, including the Americans, think that you can get rid of social jetlag by setting an alarm clock also during the weekend, so you adjust your weekend sleep to the weekdays sleep. And that is due to the Puritan background of having a bad conscience if you “sleep in”.

Even Europeans set their alarm clocks on weekends, at least sometimes.

But you understand the concept. You can understand that smoking is bad for you and still sometimes have a cigarette; if you don’t even understand it, then you are in much bigger trouble.

Are there other identifiable categories that stand out as more or less receptive to the idea of a social jetlag?

The early types don’t understand it, because they don’t have a notion of what it feels like to not be able to fall asleep early and to get up early. Under modern light conditions, most biological clocks become later and later. Only the very early types become earlier, even if they stay up beyond their biological bedtime, due to social pressure, their body clock will wake them up as early as usual. They think that late types simply should go to be earlier and then would wake up early enough for work: that is a typical early-type concept. What they don’t understand is that as little they can sleep in, late types can’t sleep early.

Speech by Roenneberg at the VELUX Daylight Symposium in 2019, in Paris

Can lighting design or building design help reduce social jetlag?

Of course. It’s being worked on, but it’s not as persuasive as I’d like it to be. There are two ways of reducing the health hazards that come with social jetlag. One is to “go camping”: that means we get a lot of daylight during the day and practically no light between sunset and sunrise. We can actually do that without moving out of our homes and setting up tents, if we design buildings so that the sunlight on the roof of buildings could be mirrored or light-piped into the ceiling of every room of the building. It must be a lot of light and it should come from above; if it comes from a side window we will have a glare and we won’t be able to work and the intensity would decline rapidly with increasing distance to the window. Artificial lights should also be spectrally adapted to the seasonal changes in sunrise and sunset. They should automatically take away most of the blue content of light after sunset, and add it back when the sun rises. This is the technological way of getting us all “camping” during industrialized life.

The other way of reducing social jetlag is to have a total flexibility of work times. Although the Coronavirus pandemics is terrible and has killed hundreds of thousands of people, the lockdown taught us that in many jobs there is no real need to be in the office at 9.00 am to keep things going.

All this does not sound easy.
I’m not saying it’s easy. I’m saying it has to be done.