Why darker days affect our sleep and mood: circadian clocks, SAD, and practical tips

Original publisher: The Conversation. This article explains how shorter daylight hours can shift our circadian clocks, influencing sleep, mood, and carbohydrate cravings. It also discusses seasonal affective disorder and the broader winter blues, and outlines practical steps to protect wellbeing including daylight exposure, regular exercise, and consistent sleep-wake times. Treatments for SAD such as cognitive behavioral therapy and antidepressants are noted, with a caution about the uncertain effectiveness of bright light therapy.

• Circadian timing is influenced by daylight exposure and can affect mood and sleep.

Overview of daylight and circadian timing

As days shorten in autumn and winter, opportunities for natural daylight decrease. The article explains that the circadian system, which governs alertness and sleep, relies on light detected by retinal cells to synchronize with the outside world. Morning daylight helps align this internal clock, while indoor work and cloudy weather can reduce light exposure, potentially shifting sleep-wake patterns and mood.

Seasonal affective disorder and sleep

About 3 percent of the UK population experiences seasonal affective disorder, a form of depression that recurs seasonally. SAD commonly begins in autumn or winter and improves in spring, with symptoms such as persistent low mood, irritability, concentration difficulties, anhedonia, increased sleep, and carbohydrate cravings. Others experience milder seasonal changes without full clinical depression.

The clock change and the evidence for mood effects

Turning the clocks back shifts an hour of daylight from the evening to the morning but does not change the total daylight. The evidence that clock changes worsen mood or trigger depression is mixed. A Danish study found more depression-related hospital contacts after autumn clock changes, while a 2025 English study reported fewer depression-related health events in the following week. These discrepancies reflect differences in outcomes and study design rather than a proven causal link.

Evolutionary and biological perspectives

One hypothesis posits that winter lethargy and energy conservation were advantageous for our ancestors facing scarce resources and harsh conditions. While intriguing, this idea remains difficult to test in modern humans, and SAD likely results from a combination of genetic susceptibility, circadian biology, and environmental factors.

Practical guidance for working with the seasons

Practical steps include spending daylight outdoors, maintaining regular exercise, and keeping consistent sleep and wake times. Social connections also help. For SAD, options like cognitive behavioral therapy and antidepressants are discussed, though evidence for specific medicines is nuanced. Bright-light therapy may help some individuals, but current guidelines note uncertainty about its effectiveness. If depressive symptoms persist, speaking with a GP and encouraging workplaces to facilitate daylight exposure can be beneficial.

Bottom line

Recognizing the biological influences of daylight on mood and energy can help people respond more effectively to seasonal changes without assuming caffeine or other quick fixes will solve everything.