Influence on the internal clock Intensive light makes you sleepy

As the intensity of sunlight increases, the amount of sleep increases

For many people, the lack of light in the winter months makes them tired and impotent. However, certain insects become drowsy when the sunlight is particularly intense. That's what researchers have discovered.

Effects of different light intensities on the day-night-rhythm

Health experts usually advise to pay attention to sufficient light, otherwise you quickly catch up with fatigue. In certain insects, on the other hand, with increasing intensity of sunlight, the amount of sleep increases, the lunch break lengthens, and the resumption of activity shifts into the evening. This is the finding of researchers from Germany and the USA, who had investigated the effects of different light intensities on the day-and-night rhythm of the fruit fly Drosophila.

People tend to get tired when it's dark. But certain insects become drowsy when the sunlight is particularly intense. (Image: karandaev / fotolia.com)

Inner clock is synchronized

"We were able to show that light of low, medium and high intensity synchronizes the internal clock in the fly's brain to the 24-hour day on different, specific paths", explained Prof. Dr. med. Charlotte Helfrich-Förster from the Julius-Maximilians-University Würzburg (JMU) in a communication.

Together with her former doctoral student dr. Matthias Schlichting, who is now researching at the Laboratory of Nobel Laureate in Physiology 2017, Michael Rosbash, at Brandeis University in Waltham, Massachusetts (USA), led the study.

The results of the team were published in the current issue of the journal "The Journal of Neuroscience".

Delayed start to the usual evening activities

The experiments show that high-intensity light significantly influences the behavior pattern of fruit flies. Noticeable was especially a significantly extended "siesta" of the Drosophila by an average of about one hour, but at least by 30 minutes.

This was accompanied by a delayed start to the usual evening activities, which were usually recorded by the flies with a delay of one to two hours.

The researchers, who are responsible for these behavioral changes at the molecular level, were also able to identify the researchers: special photoreceptors outside the retina known as Hofbauer-Buchner-Äuglein or HB eyelets.

These microscopic "auxiliary eyes" - each of them consists of just four sensory cells - have only been known for 30 years. They were discovered in 1989 by the Würzburg biologists Alois Hofbauer and Erich Buchner.

High levels of light intensify the siesta of the flies

"We were able to show for the first time that the Hofbauer-Buchner-Äuglein react to high light intensities and strengthen the siesta of the flies," said Helfrich-Förster.

The pathway they take is multi-step: via specific acetylcholine receptors, the HB eyelets act on a subset of the clock neurons, which increases their calcium levels.

As a consequence, the neurons in the back part of the fly brain release a special neuropeptide - the "pigment-dispersing factor" (PDF).

There, PDF influences the activity of other clock neurons that regulate siesta manifestation and control the onset of evening activity.

According to the scientists, this reaction to high light incidence in the Drosophila is meaningful.

"This is an essential mechanism to avoid strong, potentially harmful light during lunchtime," said the neurobiologist.

After all, high light intensities are often associated with hot summer days, which can be particularly dangerous for insects - on the one hand, because they could dry out quickly, on the other hand, because they are more easily detected in the bright light of their predators.

Comparable structures in humans

In order to rule out the possibility that not the intense light radiation, but possibly the associated heat development is the trigger for the prolonged nap, the researchers have also carried out their experiments on special, genetically modified flies.

Their internal clock is no longer responsive to temperature changes, but the flies still show the changed behavior. This suggests that the light entry is responsible for the extended nap and the late evening activity.

According to the authors, the results of their study show "surprising parallels to mammalian mechanisms".

Mammals - and humans as well - have specialized photoreceptors that sense strong light, directing their signals directly to the inner clock into a special area of ​​the hypothalamus.

Whether their activation, however, in humans leads to an extension of the siesta, is not known. The scientists think it is quite possible. (Ad)