Every autumn, something shifts. The mornings get harder. You sleep longer but wake up exhausted. Your mood dips in a way that's hard to explain to people who don't feel it. Motivation thins out. You crave carbohydrates, pull away from social plans, and count the days until spring.
If this pattern repeats itself reliably every year, there is a good chance you are experiencing Seasonal Affective Disorder. This is not a personality trait, a weakness, or something you should be able to think your way out of. It is a biological response to light deprivation — one that affects an estimated two million people in the UK each year.
What Is SAD?
Seasonal Affective Disorder is a subtype of depression that follows a seasonal pattern, almost always emerging in autumn and winter and lifting in spring. The DSM-5 classifies it as Major Depressive Disorder with a Seasonal Pattern — which is to say it is clinically recognised, well-studied, and treatable.
It is distinct from the milder 'winter blues', which affect a much larger proportion of the population (up to 20%) with lower mood and energy but without the full clinical picture of depression. SAD, by contrast, is estimated to affect around 3% of the UK population with sufficient severity to interfere significantly with daily life.
Symptoms
The symptoms of SAD overlap substantially with general depression, but with a specific seasonal character. The most common include persistent low mood that does not respond easily to positive events; fatigue and oversleeping, often sleeping two or more hours longer than in summer; increased appetite, particularly for carbohydrate-rich foods; weight gain; difficulty concentrating; withdrawal from social activity; and a loss of interest in things that would normally give pleasure.
Atypical features are common — particularly hypersomnia (sleeping too much) and hyperphagia (eating too much) — which distinguishes winter SAD from the classic presentation of depression, where insomnia and appetite loss are more typical.
What Causes It?
The underlying mechanism is well established. As the days shorten, two things happen in your brain that combine to produce the SAD presentation.
First, your body produces more melatonin. Melatonin is the hormone your brain secretes in darkness to signal that it is time to sleep. In winter, because there are more hours of darkness, melatonin is produced for longer — which creates a biological pressure toward sleep, low energy, and low mood that extends into your waking hours.
Second, reduced light exposure disrupts serotonin levels. Serotonin, the neurotransmitter most associated with stable mood, is produced in response to light and is a precursor to melatonin. Less light means less serotonin, which directly affects mood and cognitive function.
The result is a circadian system that is, effectively, running on the wrong timing. Your body clock is anchored to a seasonal signal that your modern indoor environment cannot override because most indoor lighting is far too dim and spectrally inadequate to substitute for natural daylight.
What Actually Helps?
Light therapy has been the first-line treatment for SAD since the 1980s. Clinical trials consistently show it is as effective as antidepressants for seasonal depression, and the combination of both outperforms either alone. The standard protocol is 10,000 lux of bright light, delivered within the first hour of waking, for 20 to 30 minutes per day.
Cognitive behavioural therapy adapted for SAD (CBT-SAD) is also well-evidenced and has the advantage of preventing relapse across subsequent winters even after treatment ends. For moderate to severe SAD, antidepressants — particularly SSRIs — are effective, though response rates are somewhat lower than for non-seasonal depression.
Maintaining regular sleep and wake times, getting outside during daylight hours, and keeping active are all consistently supported by the evidence. They will not resolve severe SAD on their own, but they meaningfully reduce symptom load.
The Gap in Standard Light Therapy
One important development in the science is the identification of melanopsin as the key photoreceptor driving the circadian response to light. Melanopsin is maximally sensitive to light at around 480 to 490 nanometres — a narrow band in the blue-green region of the spectrum. Standard broad-spectrum SAD lamps at 10,000 lux deliver light across the full visible range, which means only a fraction of the light output is hitting the wavelengths that actually drive the biological clock.
This does not mean existing SAD lamps do not work. They do — the evidence supports them. But it does mean the science has moved on significantly since the 10,000 lux standard was established in the 1980s, and there is now a basis for much more targeted spectral delivery. For a full breakdown of what's currently on the market, see our best SAD lamp 2026 guide.
If you think you may have SAD, speak to your GP. The condition is well recognised and treatment options are effective. For most people, light therapy is the logical first step — accessible, side-effect free, and with a strong evidence base. The key is getting enough of the right light, at the right time, consistently through the season.
