About Night Owls and Early Birds

Manuel Spitschan and his team at the Max Planck Institute for Biological Cybernetics in Tübingen are investigating how the human eye processes light stimuli and what role these play in the circadian clock.
 

Some people are considered nocturnal. Others sneeze when the sun dazzles them. Manuel Spitschan and his colleagues at the Max Planck Institute for Biological Cybernetics in Tübingen are investigating how the human eye processes light stimuli and what role they play in the internal clock, which seems to keep ticking even in complete darkness.

The sound of your cell phone alarm clock in the morning, the smell of freshly ground coffee, the feeling of the first rays of sunshine shining on your face on the way to work. Many people's everyday lives are determined by routines and recurring sensory impressions. They provide orientation, structure our day and indicate what action and feeling will come next. Light plays a decisive role in this: many people notice that they find it harder to get going in winter, feel how sunny days awaken their spirits, realize that in the evening, first our concentration and then we ourselves sink - namely into bed. So it seems reasonable to assume that it is the sun and our routines alone that give our lives a rhythm. However, there is also another clock ticking deep inside us. And this has a considerable influence on how our everyday life runs.

Insects whose stage of development depends on the phase of the moon. Plants that change their leaf position in a 24-hour rhythm. Molds that periodically form new spores even in outer space, as if they knew exactly what time it was even when they were on the other side of the earth: many processes in biology follow highly specific rhythms, just like in a strictly composed orchestral piece. An innate clock, the so-called internal clock, acts like a conductor to ensure that everything runs according to (time) plan in the musical piece of life. Environmental influences such as light help the inner clock: they provide clues as to whether the tempo of the conducting should be slowed down or sped up. However, the internal clock is not necessarily dependent on co-conductors to keep ticking away. Even in outer space, it imposes certain rhythms on us and other life forms - or even inside a bunker.

The fact that not only animals and plants, but also humans, have an internal clock was considered certain by scientists in the 20th century at the latest. However, the exact properties of the internal clock and the signaling pathways it uses to control physical processes are still a mystery to researchers today. In the 1970s, researchers at the Max Planck Institute of Behavioral Physiology wanted to find out more about it and carried out experiments that are still notorious today. Volunteers spent several weeks in an underground bunker in the absence of daylight, measuring their body temperature and observing their behavior. It turned out that a circadian rhythm was established even without daylight. The sleeping and waking phases of the test subjects therefore roughly mirrored a 24-hour day.

A reliable clock in the brain

“Circadian comes from circa and dian, meaning approximately and day,” explains Manuel Spitschan. The psychologist has a doctorate in neuroscience and is Professor of Chronobiology at the Technical University of Munich. As a research group leader at the Max Planck Institute for Biological Cybernetics in Tübingen, he investigates how light affects our internal clock. Nobody has to go into a bunker for his experiments. And the results are revealing: the human internal clock may not come close to an atomic clock. Nevertheless, it works surprisingly reliably. “Experiments in which we decouple the internal clock from the external stimulus of natural light radiation show that the clock continues to run anyway,” explains Manuel Spitschan. In the sleep laboratory, for example, test subjects sleep for an hour and a quarter and are then awake for two and a half hours. “We repeat this sleep-wake rhythm several times over a period of 40 hours.”  During a normal and uninterrupted day, adenosine accumulates in the brain. This molecule is a breakdown product of ATP (adenosine triphosphate), the energy carrier for body cells, and leads to increasing sleepiness at the end of the day. The longer you stay awake, the stronger the so-called sleep pressure becomes. “By letting our test subjects sleep again and again, we reduce this sleep pressure,” explains Manuel Spitschan. “But I wouldn't recommend anyone to live like this.” Two processes take place in the body: One controls the sleep pressure, the other the internal clock. Under natural conditions, the two interact with each other.

The experiment is therefore solely concerned with understanding the internal clock and the way it affects the body. To this end, the researchers are measuring how body temperature, the concentration of the body's own hormones melatonin and cortisol or the pupil reaction to light change throughout the day. “The rhythm of the internal clock that we usually measure is not exactly 24 hours, but perhaps 23.5, 24.2 or 24.5 hours,” says Manuel Spitschan. Some people's inner day is therefore a little too short. Others, on the other hand, tend to go to bed a little later every day. On average, the human biorhythm approaches the natural day length of 24 hours.

These experiments show that the internal clock controls countless processes in the body and not only has an influence on how awake or tired we feel. For example, it causes our body temperature to fluctuate periodically throughout the day, regulates when and how many stress hormones are released and causes our metabolism to ramp up or down at certain times. “We can observe many different circadian rhythms in our experiments - for example with regard to visual performance or our attention,” explains Manuel Spitschan. “For example, we can concentrate differently at different times of the day depending on our internal clock. The immune response also appears to vary circadian, as does muscle performance. Overall, it seems that almost all human physiological functions are dependent on circadian rhythms.”

The existence of these rhythms is now completely undisputed. However, the exact signaling pathways that control them are still unknown in many cases. However, researchers were able to answer one key question a few years ago: the location of the internal clock in the human body. “The suprachiasmatic nucleus, or SCN for short, is a small bundle of nerves in the brain where the two optic nerves cross,” explains Manuel Spitschan. “Studies have shown that although the SCN is not the only one, it is apparently the central clock generator for a number of physiological processes.” Among other things, the rice grain-sized structure has been shown to send signals to the pineal gland - another very small brain region that produces the hormone melatonin. When the pineal gland releases melatonin, the body receives the signal that it is time to sleep. “In laboratory experiments, we see that the release of melatonin in the human body also follows a certain pattern that repeats roughly every 24 hours. This shows that the length of the period is determined by the molecular processes of the cells.” In other words, even without light signals from the environment, the internal clock periodically instructs the pineal gland to produce melatonin. Whether someone is an early riser or a night owl is therefore a matter of genetics. However, the internal clock should not be thought of as a completely independent structure: It has a certain flexibility and can shift its hands a little forward or backward depending on environmental influences. However, such corrections tend to take place slowly. After long flights, for example, the internal clock adjusts to the new time zone with the help of light stimuli. However, this can take a few days. To be more precise: about one day for every hour of time difference.

The internal clock is adaptable

Although light as an external stimulus cannot change the period of around 24 hours, it does have a calibrating effect on the internal clock. Manuel Spitschan and his colleagues in Tübingen are researching exactly how this works. “In our studies, we are investigating how different light stimuli are processed in the brain and how the internal clock reacts to them.” The researchers expose test subjects to light of different intensities and wavelengths in their sleep laboratory and again measure precisely those parameters that change periodically in time with the internal clock. As the results from the sleep laboratory are often difficult to transfer to everyday life, the chronobiologists also carry out field studies. “In a long-term study, for example, we are currently measuring sleep under natural conditions,” says Manuel Spitschan. “The test subjects are given a small EEG device that measures their brain waves. They attach it to their heads at home in the evening, and we then use it to record how different sleep phases alternate over the course of a year - for example, how long the deep sleep phases are.”

In further experiments, the scientists are measuring how much light people are exposed to in their everyday lives. The background to this is that fundamental data on this has never been collected before. However, such data and corresponding long-term studies are urgently needed in order to quantify the effects of light in everyday life and to be able to make empirically based health recommendations. Over the course of the day, people are exposed to different light sources: daylight, electric light, light from displays and combinations of these. In order to measure daily exposure to light, the researchers at the institute developed a series of electronically implemented custom-made devices - glasses, a necklace and a bracelet. Test subjects can wear these on their bodies in everyday life without much effort. “They each have small sensor chips attached to them that record the light intensity over the course of the day,” explains Manuel Spitschan. Among other things, the researchers use them to measure the amount of light that test subjects in Germany, Spain, Sweden, Ghana, the Netherlands and Turkey are exposed to as they go about their everyday activities.

What results of his basic research has Manuel Spitschan found particularly enlightening so far? “At the turn of the millennium, it emerged that there is another cell type in the retina in addition to the rods and cones, the melanopsin-containing ganglion cells,” says the researcher. “This photoreceptor has a significant influence on the internal clock.” When these cells process light stimuli, they apparently send a signal to the suprachiasmatic nucleus and thus set the internal clock back a little. The brain thus receives the reference information that it must still be daytime and does not release melatonin for the time being. In order to clarify whether the cones of the eye can also send signals to the internal clock, Manuel Spitschan and his colleagues designed several elaborate experiments. “We generated special light stimuli that only trigger the cones in the eye. It turned out that this has no effect on melatonin production.” This leads to the conclusion that not all light is the same. Only certain light stimuli are relevant for the internal clock - namely those that act on melanopsin-containing ganglion cells. “This is actually a major gain in knowledge: it means that we can leave the cones out of our experiments.” And there is more. Experiments show that the ganglion cells are particularly sensitive to blue light with a wavelength of 490 nanometers. Artificial light on cell phone screens therefore certainly has an effect on the brain. Above all, however, it depends on the brightness of the light source. Simply dimming the cell phone screen reduces the impact on the internal clock more than a blue filter.

The studies of Spitschan's working group on the photic sneeze reflex are probably less consequential for research, but all the more entertaining. The background to this is that around 20 to 30 percent of all people sneeze compulsively when exposed to bright light. Manuel Spitschan himself, by the way: “As a scientist, I naturally want to know what happens in the brain.” One of the students at the institute took part in this research, documenting his sneezes for a month. On average, he counted just under three a day. “The frequency of photic sneezing depends on the time of year - in summer, of course, there is much more daylight than in winter. My theory is that the reflex has something to do with the melanopsin-containing ganglion cells. However, we have not yet been able to explain the mechanism.”

Dimmed displays protect the internal clock

Research into exactly how light affects the human body and brain is still in its infancy. Nevertheless, chronobiologists around the world agree that our exposure to light has a decisive impact on human health and quality of life. To make more people aware of this, Manuel Spitschan's colleagues will be touring Germany with a mobile research laboratory from May 2025. “For example, you can use a questionnaire to determine your own chronotype or observe your own pupil reactions on a monitor while you are exposed to light stimuli,” explains Manuel Spitschan. The aim is to arouse more interest in chronobiological correlations and to disseminate research findings more widely. Persistent misconceptions have caused unnecessary suffering and unjustified criticism among the population for decades. “For example, people believe that they are lazy because they can't get up early. But this is often biological. Bringing this knowledge more into society can have a relieving effect.”