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Seasonal affective disorder and its therapies

Edexcel IAL Psychology · WPS02 Biological Psychology, Learning Theories and Development

Seasonal affective disorder (SAD) is depression that returns at the same time each year, usually autumn and winter. The specification requires at least two therapies, including light therapy (spec 3.1.5 and 3.1.6).

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SAD as an infradian rhythm

SAD follows a yearly (circannual) pattern, so it is classed as an infradian rhythm. Symptoms include low mood, low energy, oversleeping and carbohydrate cravings.

Explanations link it to reduced daylight:

Melatonin: longer nights mean more melatonin, adding to tiredness.
Serotonin: less light may reduce serotonin activity, lowering mood.
Circadian phase shift: short days may delay the body clock (Lewy et al., 2006, found depression ratings correlated with circadian phase in winter depression).

Therapy 1: light therapy

The person sits near a light box of about 10,000 lux, usually for 20 to 30 minutes a day, often in the morning. Bright light suppresses melatonin and may shift the body clock earlier.

Evidence: Eastman et al. (1998), in a trial of 96 people, found response rates of 61% for morning light, 50% for evening light and 32% for a placebo.

Evaluation: quick, non-invasive and few side effects, but must be used daily, mild side effects can occur, and some people with eye conditions or bipolar disorder need medical advice first.

Therapy 2: CBT, and antidepressants

Cognitive behavioural therapy (CBT) helps people identify and change unhelpful thoughts (such as wanting to 'hibernate') and increase activity. Rohan et al. (2015), with 177 adults, found six weeks of CBT matched light therapy for an episode. Two winters later (Rohan et al., 2016), depression returned in about 27% of the CBT group and about 46% of the light therapy group, suggesting CBT lasts longer.

Antidepressants (SSRIs) stop serotonin being reabsorbed, raising its level in the synapse. Lam et al. (2006) compared light therapy and fluoxetine in 96 patients: both had a 67% response rate, but light worked faster and fluoxetine had more side effects.

Comparing the therapies

In an exam answer, compare therapies on speed, side effects, relapse, cost and who they suit, and reach a judgement. A combination (for example light therapy for quick relief plus CBT to prevent relapse) is often argued to be best.

Therapy	Strength	Weakness
Light therapy	Fast, few side effects, good evidence	Daily commitment; symptoms may return when stopped
CBT	Skills last; lower relapse (Rohan et al., 2016)	Takes weeks; needs a trained therapist and effort
SSRIs	Effective for more severe cases	Side effects; takes weeks to work

Worked example

Using Eastman et al. (1998), how much higher was the response rate for morning light than for placebo?

Morning light: 61%. Placebo: 32%.
61 - 32 = 29 percentage points.

Answer: 29 percentage points higher.

Key idea: SAD: winter depression linked to melatonin, serotonin and body clock delay. Light therapy (about 10,000 lux) works quickly; CBT has lower relapse; SSRIs raise serotonin.

Check yourself

SAD is classed as which kind of rhythm?
- Infradian
- Circadian
- Ultradian
- Ninety-minute
Show answer
Infradian. A yearly cycle is longer than 24 hours: infradian.
About how bright (in lux) is a typical light box for SAD?
Show answer
10000 lux. About 10,000 lux.
In Eastman et al. (1998), morning light had a 61% response rate and evening light 50%. What is the difference in percentage points?
Show answer
11 percentage points. 61 - 50 = 11 percentage points.
What did Rohan et al. (2016) find two winters after treatment?
- Light therapy had no effect at all
- SSRIs worked best
- Everyone relapsed equally
- Depression returned less often after CBT than after light therapy
Show answer
Depression returned less often after CBT than after light therapy. About 27% relapse after CBT versus about 46% after light therapy.
Rohan et al. (2016): relapse was about 46% after light therapy and about 27% after CBT. How many percentage points lower was relapse after CBT?
Show answer
19 percentage points. 46 - 27 = 19 percentage points.
How do SSRIs help SAD?
- They increase melatonin
- They block daylight
- They destroy the SCN
- They stop serotonin being reabsorbed, so more stays in the synapse
Show answer
They stop serotonin being reabsorbed, so more stays in the synapse. Selective serotonin reuptake inhibitors raise synaptic serotonin.
Explain how light therapy is thought to reduce the symptoms of SAD.
Show answer
Bright light from a light box, often in the morning, suppresses melatonin so the person feels less sleepy, and may reset the delayed body clock and raise serotonin activity, improving mood.. Name the mechanism.

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Last updated 8 November 2026. Spotted a mistake? Tell us.