Researchers have found that the length of daylight hours influences the level of opioid receptors in brown adipose tissue. As daylight hours shorten, receptor activity increases. A similar phenomenon also occurs in the brain. Both phenomena help humans and animals adapt to seasonal changes.

The Importance of Brown Fat

As the season grows darker and colder, animals’ brown fat begins to grow. This tissue generates heat efficiently and quickly and regulates appetite. Brown fat is also present in humans. Brown fat is responsible for burning energy and converting it into heat. This process is called thermogenesis and helps the body regulate its temperature, especially in cold conditions. Brown fat contains a particularly high number of mitochondria—the “powerhouses” of cells—which give it its characteristic dark color. In humans, brown fat is most prominent in newborns, as they are not yet able to regulate their body temperature effectively on their own. In adults, it is present only in small amounts but continues to play a role in metabolism and energy expenditure.

Brown adipose tissue not only plays an important role in heat production but also appears to be closely linked to the body’s biological rhythms. Studies show that its activity is influenced by external cues such as light, temperature, and day length. These factors control the circadian rhythm—the body’s internal clock, which regulates sleep, hormone production, metabolism, and energy expenditure. Brown fat possesses its own molecular clock mechanisms and follows diurnal fluctuations that influence its ability to produce heat and utilize energy. Researchers therefore suspect that the tissue is part of a complex network through which the organism adapts to daily and seasonal changes. Changes in the duration of daylight could not only influence the activity of brown fat but also affect metabolism, appetite, body weight, and possibly even mood and well-being. As a result, brown fat is increasingly becoming the focus of chronobiology, which studies how biological processes are controlled by internal and external time structures.

New Therapies for Obesity

The importance of brown adipose tissue for health is increasingly becoming the focus of research. Unlike white adipose tissue, which stores excess energy and is more prevalent in overweight individuals, brown fat cells burn energy to produce heat. In doing so, they not only help maintain body temperature but also influence the body’s overall energy balance. Scientists therefore suspect that greater activation of brown fat could help combat obesity and related metabolic diseases.

A study by the University Hospital of Bonn demonstrates just how much interest there is in this tissue. The researchers identified the protein EPAC1 as a key regulator of the growth and activity of brown adipose tissue. Experiments showed that EPAC1 promotes the formation of brown fat cells and can even help so-called beige fat cells develop within white adipose tissue. These cells have properties similar to those of brown fat cells and can also burn energy. The scientists were also able to demonstrate that this signaling pathway is active not only in mice but also in human fat cells.

In the long term, the researchers hope to use these findings to develop new therapies for obesity. Since diet and exercise alone are often insufficient for sustainable weight loss, scientists are searching for ways to specifically increase the body’s energy expenditure. Brown fat is considered a particularly promising approach in this regard, as it can release excess energy in the form of heat. The Bonn findings thus illustrate that brown fat is far more than just a tissue for thermoregulation—it could also play an important role in the treatment of metabolic diseases in the future. Against this backdrop, interest is also growing in the biological factors that control brown fat activity. These include not only temperature and diet, but apparently also the duration of daylight and the body’s internal biological rhythms.

How Daylight Affects Brown Fat

The study conducted by the Turku PET Centre in Finland addressed precisely this question. The research builds on findings in chronobiology, which indicate that numerous metabolic processes are controlled by internal biological clocks. These clocks are guided, among other things, by external signals such as light and darkness and help the organism adapt to daily and seasonal changes. Since brown fat plays an important role in heat production and energy expenditure, the researchers hypothesized that its activity might also be influenced by seasonal changes. Of particular interest were the mu-opioid receptors, which are primarily known for their role in pain processing, reward perception, and emotional processes, but are increasingly being linked to the regulation of metabolic processes.

The researchers observed that shorter daylight hours influence signal transmission via the opioid receptors in the brown fat of animals. As the amount of light decreases, the concentration of opioid receptors increases. The observation was made in rats living in an artificial environment that mimicked seasonal changes in daylight duration. The results suggest that brown adipose tissue is sensitive to seasonal changes and may be part of a broader biological system that adapts energy balance to varying environmental conditions.

New Breakthrough in Opioid Receptor Research

The researchers consider the results particularly noteworthy because mu-opioid receptors have previously been studied primarily in connection with brain functions. The fact that seasonal changes in these receptors have now also been detected in brown adipose tissue opens up new perspectives for understanding the interactions between metabolism, energy balance, and biological rhythms. Since brown adipose tissue plays a central role in heat production and calorie expenditure, the activity of opioid receptors could help the body adapt to different environmental conditions and seasons.

Professor Anne Roivainen of the Turku PET Centre explains that this is the first time that concentrations of mu-opioid receptors in peripheral regions of the body have been studied using positron emission tomography (PET). “The findings underscore that mu-opioid receptors influence the seasonal activity of brown adipose tissue. Future studies should further investigate whether mu-opioid receptors in brown adipose tissue are directly linked to the tissue’s energy expenditure,” says Roivainen.

Opioid receptors are binding sites on cells through which the body’s own opioids, such as endorphins, exert their effects. These neurotransmitters are involved, among other things, in regulating pain perception, reward, motivation, and emotional well-being. In recent years, however, research has increasingly found evidence that the opioid system not only influences the brain but also plays a key role in metabolism and energy balance. The seasonal changes now observed could therefore be part of a broader adaptive mechanism through which mammals adjust to the changing demands of summer and winter.

Furthermore, scientists have long suspected that changes in the opioid system might be linked to seasonal mood fluctuations. Disturbances in opioid receptor function have already been associated with depression, anxiety disorders, and eating disorders. Seasonal affective disorder, which often manifests as winter depression, increased appetite, and altered sleep patterns, may also be partially related to changes in these signaling pathways. However, it remains unclear whether the now-demonstrated seasonal fluctuations in mu-opioid receptors in the brain and brown adipose tissue actually contribute to such mood effects, and this must be investigated in further studies.