You might think that summer is healthier. The sun is shining, we get plenty of vitamin D, and the days are long. However, research findings from the University of Copenhagen suggest that winter eating habits are better for our metabolic health than summer eating habits, at least if you are a mouse.

Researchers studied the metabolism and weight of mice exposed to both “winter light” and “summer light.” “We found that even in non-seasonal animals, differences in light hours between summer and winter lead to differences in energy metabolism. In this case, that’s body weight, fat mass, and liver fat content,“ says Lewin Small, who conducted the research as a postdoctoral fellow at the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen. He adds, ”We saw this most notably in mice exposed to winter light conditions. These mice gained less body weight and had lower obesity levels. They ate more rhythmically over a 24-hour period. This then led to benefits for metabolic health.” The study is the first of its kind to examine the influence of daylight hours on the metabolism of mice, which are not considered seasonal animals because, like humans, they do not breed only during certain seasons. Animals that breed during specific seasons gain weight before the breeding season to conserve energy reserves.

Daylight Hours Influence Metabolism

Daylight plays a central role in human metabolism because it significantly controls the internal biorhythm. When light hits the eyes, the body sends signals to the internal clock in the brain, which then regulates various hormones. In the morning in particular, bright daylight ensures that the production of the sleep hormone melatonin is reduced. At the same time, cortisol levels rise naturally, which makes the body awake and active and stimulates energy metabolism. A well-balanced light and hormone rhythm helps the metabolism to work evenly, supports blood sugar regulation, and keeps fat burning on track. Too little daylight, as is often the case in winter or during long periods indoors, can disrupt this process. The body remains in a kind of rest mode for longer, fatigue increases, and the metabolism slows down.

Daylight also influences the production of serotonin, a hormone that is important for both mood and appetite. With sufficient light, serotonin levels are higher, which boosts general well-being and can prevent food cravings. A lack of light reduces serotonin activity, causing the body to crave energy-rich, carbohydrate-rich foods more often, which in turn can affect body weight and metabolism. In addition, sunlight plays an important role in the formation of vitamin D, which is involved in various metabolic processes, including sugar and fat metabolism and muscle strength. A vitamin D deficiency, which is more common in low-light months, can therefore indirectly contribute to a slower metabolism. Overall, it is clear that daylight is much more than just a factor for good vision. It acts as a pacemaker for hormones, energy balance, sleep quality, and appetite regulation. Regular exposure to sufficient daylight, especially in the morning, not only boosts your mood but also supports a healthy, active metabolism.

Differences in Light Intensity Between Summer and Winter could Influence Our Eating Habits

The researchers were inspired to conduct the study due to the significant differences in daylight hours in different regions of the world. “We are investigating the influence of the time of day on aspects of metabolism such as exercise, obesity, and diabetes. However, most studies investigating this relationship assume that the length of day and night is the same throughout the year,” says Lewin Small. Therefore, they wanted to find out what the seasonal differences in light mean for metabolism. Most people in the world live with a light difference of at least two hours between summer and winter. “I come from Australia, and when I moved to Denmark, I wasn’t used to the big difference between summer and winter and was interested in how this might affect circadian rhythms and metabolism,” says Lewin Small, adding: “So we exposed lab mice to different light conditions representing different seasons and measured markers of metabolic health and circadian rhythm in these animals.”

Since the research was conducted using mice as test animals, it cannot be assumed that the same applies to humans. “This is proof of principle. Do differences in hours of light affect energy metabolism? Yes, they do. Further studies in humans could show that changing our exposure to artificial light at night or natural light throughout the year could be used to improve our metabolic health,” says Juleen Zierath, professor at the Novo Nordisk Center for Basic Metabolism Research (CBMR) and lead author of the study. Lewin Small adds that these new findings are important for understanding how eating habits are influenced by light and seasons, which could help us understand why some people gain more weight or whether people gain more weight at certain times of the year. Differences in light intensity between summer and winter could influence our feelings of hunger and when we feel hungry during the day.

What and How Much We Eat Can Alter Our Internal Clock and Hormone Responses

Previous research has shown that glucocorticoid hormones such as cortisol regulate sugar and fat levels differently over a 24-hour period, depending on the time of day or night, food intake and fasting, rest and activity. A study conducted on mice found that the time-of-day-dependent metabolic cycle is altered by a high-calorie diet. Since glucocorticoids are often used to treat inflammatory diseases, these Molecular Cell published results suggest that lean and obese patients may respond differently to steroid therapy. Finally, the biological function of the daily rhythm of hormone secretion (high before waking and eating, low during sleep and fasting) and the daily cycles of sugar and fat storage or release by the liver are demonstrated.

Every cell in the human body is controlled by an internal clock that follows the 24-hour circadian rhythm. It is synchronized with the natural day-night cycle mainly by sunlight, but also by social habits. In a healthy system, glucocorticoid stress hormones are produced by the adrenal gland every morning. The release of glucocorticoids peaks before we wake up and causes the body to use fatty acids and sugar as energy sources so that we can begin our daily activities. When the circadian rhythm is disrupted (e.g., by shift work or jet lag) and/or when glucocorticoid levels change (e.g., due to Cushing’s syndrome or long-term clinical use), this can lead to profound metabolic disorders—such as obesity, type 2 diabetes, and fatty liver disease. The researchers’ goal was therefore to understand the significance of these daily peaks in stress hormone release, the influence of these hormones on our “internal clock,” and their role in daily metabolic cycles.

Metabolic Effects of Glucocorticoids in the Liver

To investigate the metabolic effects of glucocorticoids in the liver, the researchers characterized the activity of their receptor, known as the glucocorticoid receptor, using novel high-throughput techniques. They analyzed the livers of mice every four hours during the day and night. The mice were either in a normal state or were fed a high-fat diet. They then used state-of-the-art technologies from the fields of genomics, proteomics, and bioinformatics to show when and where the glucocorticoid receptor exerts its metabolic effects. The researchers investigated the effects of daily bursts of glucocorticoid release in the 24-hour cycle of liver metabolism. They were able to show how glucocorticoids regulate metabolism differently during fasting (when the mice are asleep) and during feeding (when they are active) through time-dependent binding to the genome. In addition, they showed how most of the rhythmic gene activity is controlled by these hormones. When this control is lost (in so-called knockout mice), blood sugar and fat levels are impaired. This explains why the liver regulates blood sugar and fat levels differently during the day and at night.

Since the glucocorticoid receptor is a commonly used target molecule in immunotherapy, they next investigated its genomic effects after injection of the drug dexamethasone, a synthetic glucocorticoid that also activates this receptor. “With this experiment,” explains Dr. Fabiana Quagliarini, “we found that the response to the drug was different in obese mice than in lean mice. This is the first time that diet has been shown to alter the hormonal and drug responses of metabolic tissues.”

Important Findings for Chronomedicine and the Treatment of Metabolic Diseases

Glucocorticoids are a group of natural and synthetic steroid hormones, such as cortisol, which are primarily produced in the adrenal cortex. They follow a strict daily rhythm in the body. Cortisol levels are highest in the morning to activate the body and decline throughout the day. This rhythm is controlled by the internal clock and can be influenced by light, stress, or sleep patterns. Glucocorticoids have strong anti-inflammatory and immunosuppressive properties that can be used to control the activity of the immune system. For this reason, they are widely used in medicine. The main disadvantage is that glucocorticoids also cause serious side effects due to their ability to modulate sugar and fat metabolism: patients can develop obesity, hypertriglyceridemia, fatty liver, high blood pressure, or type 2 diabetes.

“Understanding how glucocorticoids control the 24-hour cycle of gene activity in the liver and thus blood sugar and fat levels provides new insights for chronomedicine and the development of metabolic diseases. We were able to describe a new connection between lifestyle, hormones, and physiology at the molecular level, which suggests that overweight people may respond differently to daily hormone release or to glucocorticoid medications. These mechanisms form the basis for the development of future therapeutic approaches,” emphasizes Prof. Henriette Uhlenhaut.