For many people, shorter days bring a dip in mood, but for some the change goes much deeper. Seasonal affective disorder, known by the acronym Sad since the 1980s, is a form of depression that returns each winter and is far more intense than the ordinary gloom many people feel when the light fades. Although it affects only a small share of the population, it can be debilitating, disrupting work, sleep and relationships for weeks at a time.
In a video piece for the Guardian, science correspondent Ian Sample speaks with Russell Foster, a circadian neuroscientist at the University of Oxford, about what happens inside the brain when daylight becomes scarce. Foster's work centres on the body's internal clock, the biological system that regulates sleep, hormones and mood in step with the cycle of day and night. When that rhythm is thrown out of alignment by reduced light, he explains, the knock-on effects can reach into emotional health.
Foster sets out the leading explanations for why some people are more vulnerable than others, and what the evidence suggests about treatment. Light exposure is a central part of that discussion, since the same light-based approaches are used to counter the condition. Yet the broader picture remains incomplete. Scientists have not yet pinned down the exact biological mechanism behind seasonal affective disorder, and the most effective approach to treating it is still debated in the research.
The piece, published in November 2026, is a reminder that winter depression is a genuine medical condition rather than simply a matter of mood. For those who experience it, the open questions mean that treatment often involves trial and adjustment, and that understanding the role of the body's clock may matter as much as the season itself.