Tired, irritable and craving carbs? Why darker days affect our bodies and brains

As the clocks go back in October, millions of Britons enjoy an extra hour of sleep at night. However, the evening darkness may quickly fade, making that brief respite feel like a distant memory. With the arrival of autumn, you may feel unusually fatigued, have trouble focusing, or crave carbohydrate-rich comfort foods. Getting out of bed becomes challenging, and the idea of staying on the couch all night seems more appealing than socializing.

While some may attribute this to the winter blues, shorter days can indeed influence our sleep and mood. The clock change, observed in numerous countries, doesn't diminish daylight hours; instead, it shifts an hour of it from the evening to the morning. However, it coincides with shorter days, during which some individuals struggle to obtain sufficient natural light.

About 3% of the U.K. population experiences a form of depression known as seasonal affective disorder (SAD) during winter. Symptoms of SAD include persistent low mood, irritability, difficulty concentrating, and a loss of interest in activities they once enjoyed. This can also manifest as increased sleep, appetite, and cravings for carbohydrates, all of which can impact work performance and relationships.

For many, these symptoms may not meet the criteria for clinical depression, while some notice changes in sleep without significant mood alterations. Our circadian system, or internal body clock, regulates alertness and sleepiness. Light-sensitive cells in the retina detect environmental light and signal to the brain's central clock, coordinating daily rhythms, including melatonin secretion, a sleep-associated hormone.

Bright morning daylight aids in synchronizing this clock with the external environment. However, someone who commutes before sunrise, spends their day indoors, and returns home after sunset might receive relatively little natural light. Even on overcast days, outdoor light is typically brighter than typical office lighting. Reduced daylight exposure can impact circadian timing and potentially contribute to seasonal changes in sleep and mood, though the underlying causes of SAD are not fully understood.

Research also indicates variations in sleep patterns with the seasons. A 2025 study utilizing sleep-tracking data from over 116,000 adults found that individuals in the Northern Hemisphere slept approximately 15 to 20 minutes longer in winter compared to summer. These differences remained consistent despite the presence of artificial lighting and relatively fixed work hours.

Does resetting the clocks exacerbate the situation? The evidence is inconclusive. A Danish study reported an increase in hospital contacts for depression following the fall change, while a 2025 study in England indicated fewer recorded depression-related health events in the subsequent week. These studies measured different outcomes, and neither conclusively proves that changing the clocks contributes to seasonal depression.

Evolutionary biology provides one potential explanation for winter lethargy. For our ancestors in temperate climates, winter entailed colder conditions and less predictable food supplies. Venturing into harsh weather to search for food could be perilous, so conserving energy might have conferred a survival advantage. Some researchers propose that the increased sleep, appetite, and weight gain observed in certain cases of seasonal depression may reflect an inherited tendency to conserve energy when food is scarce.

While an intriguing hypothesis, testing its validity in modern humans is challenging. We cannot conclude that contemporary humans possess an automatic, evolutionarily programmed slowdown in metabolism whenever daylight diminishes. Seasonal depression is complex, with genetic susceptibility, circadian rhythms, and environmental factors potentially contributing.

Nevertheless, our sensitivity to seasonal light prompts questions about modern working life. Most workplaces expect consistent productivity year-round, despite varying opportunities to spend time outdoors between July and November. Practical steps that may help include getting outside during daylight, engaging in regular exercise, maintaining consistent sleep and wake times, and fostering social connections.

For SAD, treatment options encompass cognitive behavioral therapy and antidepressants. Research on specific medications requires careful interpretation. A small study involving 37 patients suggested that agomelatine might improve symptoms, while a Cochrane review found that bupropion could aid in preventing recurrence in individuals with a history of winter depression.

However, the latter evidence does not establish its effectiveness for treating an ongoing episode. Bright-light therapy may also benefit some individuals, although NICE states that the evidence of its effectiveness remains uncertain. Anyone experiencing persistent depressive symptoms should consult a general practitioner rather than assuming that fatigue is solely due to winter.

Workplaces could also assist by facilitating outdoor time during daylight hours. As the evenings grow longer, recognizing the biological influences on mood and energy may help us respond more effectively, without relying on coffee as a sole solution.