How Autumn Changes the Body: Fatigue, Weight and Daylight

Shorter days and falling temperatures at the start of autumn bring measurable changes to sleep, energy and body weight, according to research examined by scientists studying seasonal effects on human health. Researchers say the shift in daylight, rather than darkness alone, plays the central role in why many people feel more tired as the season turns.

Scientists writing in the Journal of Physiological Anthropology found that the amount of daylight a person receives influences the body's internal clock, the biological system that governs the sleep-wake cycle. As daylight hours shrink, the body releases more melatonin at night, a hormone involved in regulating sleep. Bright artificial light in the evening can have the opposite effect, pushing the internal clock later and making it harder to fall asleep.

The intensity, timing and duration of light exposure all determine how strongly it affects the body, researchers noted. A study of Swedish office workers tracked changes in daylight exposure across the year alongside daily patterns of melatonin and cortisol, the stress hormone that follows a pronounced daily rhythm and plays a role around the time people wake up.

Findings from Sweden, a country farther north than Germany, cannot be applied directly to German conditions, but they point to a broader pattern: daylight provides key signals that help regulate the body's internal clock. Fatigue can set in earlier when daylight hours shrink, and later in the evening when artificial light remains bright, according to the research.

Body weight also tends to shift with the seasons, though not as dramatically as commonly assumed, researchers have found. A study of 195 adults in the United States tracked weight changes between the Thanksgiving holiday in late November and the New Year. On average, participants gained 0.37 kilograms during that period. Later in winter, their weight was on average 0.48 kilograms higher than it had been in early autumn.

Those figures are considerably smaller than the two to three kilograms of "winter weight" often cited in popular belief, the research team wrote in the New England Journal of Medicine. The numbers reflected averages across the study group. Many participants gained weight during the winter months, but others lost weight over the same period, the researchers noted.

Separate weight data collected from seven countries, including Germany, showed similar seasonal patterns, according to another study. Body weight tended to be lowest in summer months and highest during colder periods of the year, researchers found.

A further study following participants over more than two decades recorded the same pattern, with weight peaking in winter and dropping to its lowest point in summer. The researchers who conducted that analysis, published in the European Journal of Clinical Nutrition, cautioned that the data do not prove the season itself causes the weight change.

Instead, the team suggested a combination of behavioral factors is more likely responsible. People tend to be less physically active during winter months, may eat more energy-dense food, attend more social gatherings and celebrations, and spend more time indoors when temperatures drop, the researchers wrote.

Winter weight gain is possible under these circumstances, according to the study, but it is not an unavoidable biological process tied directly to the changing season.

Taken together, the research on both fatigue and weight gain during autumn and winter points to a mix of biological and behavioral influences rather than a single cause. Daylight exposure affects hormone levels tied to sleep and alertness, while seasonal shifts in activity, diet and social habits appear to drive much of the weight fluctuation observed across different studies and countries.