What causes seasonal affective disorder?

SAD is thought to result from a combination of reduced light exposure affecting circadian rhythms, overproduction of melatonin (the sleep hormone) in response to longer nights, and reduced serotonin availability linked to lower light levels. People with SAD show altered serotonin transporter activity in winter. Genetic factors influence vulnerability — SAD runs in families. It affects an estimated 2–3% of the UK population as full SAD and a further 20% as sub-syndromal SAD (winter blues), with significantly higher rates in northern latitudes.

How effective is light therapy for SAD?

Light therapy (10,000 lux broad-spectrum light, used for 20–30 minutes each morning) is the most evidence-supported treatment for SAD, with response rates of 60–80% in clinical trials. It should be started in early autumn before symptoms develop for best prevention. Effects are typically apparent within one to two weeks. Light therapy is well-tolerated, with mild side effects including headache, eyestrain, and occasionally agitation or hypomania in susceptible individuals. It is available without prescription in the UK.

Are there supplements that help with SAD?

Vitamin D deficiency is common in northern latitudes in winter and may contribute to low mood, fatigue, and cognitive symptoms that overlap with SAD. Supplementation is appropriate for most people in the UK during winter months. Omega-3 fatty acids have anti-inflammatory and mood-relevant properties. St John's Wort has evidence for mild-to-moderate depression and may benefit seasonal low mood, though it interacts with many medications. Saffron extract has emerging evidence for mood support. These supplements are adjuncts, not replacements for light therapy or medical treatment of significant SAD.

Can lifestyle changes prevent SAD symptoms?

Maximising natural light exposure — particularly in the morning — is the most impactful lifestyle measure: outdoor walks within 2 hours of waking, keeping window areas clear, and using dawn-simulating alarm clocks. Maintaining consistent sleep-wake timing year-round supports circadian stability. Regular exercise, particularly outdoors, has robust evidence for mood benefit. Social connection and maintaining engaging activities throughout autumn and winter (rather than withdrawing, which worsens SAD) are important protective behaviours. These work best as prevention started before symptoms emerge.

Mental Healthmoderate Risk

Seasonal Affective Disorder

Seasonal affective disorder (SAD) is a recurrent pattern of depression that typically emerges in autumn and winter as daylight decreases, affecting mood, energy, appetite, and sleep. Light therapy, vitamin D, physical ac

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Reviewed by Dr. Ava Gardner · Integrative Medicine Researcher26 March 2026Emerging evidence

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Significantly increased need for sleep (hypersomnia)
Strong cravings for carbohydrates and comfort foods
Weight gain through the winter months
Fatigue and low energy despite sleeping more

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