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
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.