Seasonal affective disorder (SAD) is a seasonal pattern modifier to recurrent major depressive disorder. Despite cognitive behavioural therapy (CBT) having a strong evidence base of efficacy for depression, little research exists assessing CBT for SAD, especially in the acute phase of depression during winter months. The aim of this study was to determine the efficacy of CBT for acute SAD in adults. Eligible randomised controlled trials (RCTs) testing the efficacy of CBT on depression symptoms in adults with SAD were included. Depression outcomes were assessed using the Revised Cochrane Risk-of-Bias Tool for Randomized Trials. A meta-analysis using a fixed effects model was conducted to assess the effects of CBT on depression symptoms compared with light therapy (LT) at post-intervention and 1–2 years follow-up. Narrative synthesis was used for recurrence and remission rates. Three RCTs and two follow-up papers met the inclusion criteria. All RCTs measured efficacy of group-CBT for acute SAD and compared to LT. There was substantial variation in risk of bias for all outcomes across the trials. Three RCTs (n=220 participants) were included in the meta-analysis that found CBT was effective in reducing depressive symptoms compared with LT at 1–2-year follow-up post-intervention [MD=–4.5, 95% confidence interval (CI) (–6.88, –2.12), p<0.05]. There was no difference between CBT and LT at immediate post-intervention. Group-CBT appears equivalent to LT in treating acute SAD in adults at post-intervention, but appears more effective at long-term follow-up. The findings should be taken with caution due to few included studies and variation in risk of bias across studies.

Key learning aims

1. (1) Previous research into CBT and seasonal affective disorder has focused primarily on delivery of CBT during the non-acute phase of SAD, typically in non-winter months.

2. (2) There are limited high quality randomised controlled trials testing the efficacy of CBT for seasonal affective disorder in the acute phase during winter months.

3. (3) It appears that group-CBT for SAD is superior to LT at 1–2 years follow-up.

Seasonal affective disorder (SAD) is characterised by recurrent episodes in autumn and winter of depression, hypersomnia, augmented appetite with carbohydrate craving, and weight gain, and can be successfully treated with bright light. Circadian rhythm hypotheses (summarized in) have stimulated research into the pathophysiology of SAD, postulating that:

1. 1.The illness is a consequence of delayed phase position,

2. 2.It is correlated with diminished circadian amplitude, or

3. 3.It results from changes in the nocturnal duration between dusk and dawn e.g. of low core body temperature or melatonin secretion. Light is considered to act directly on the circadian pacemaker (‘Process C’) and not on sleep dependent processes (‘Process S’). Thus successful treatment of SAD must act via mechanisms within known retinohypothalamic pathways. Conversely, emergence of SAD symptoms may reflect inappropriate neurobiological response to decreasing daylength.

Since Rosenthal et al published their first study about the effects of light treatment on mood in patients suffering from winter depression, many research centres have become interested in the study and treatment of seasonal affective disorders (SAD), one of them being the department of Biological Psychiatry in Groningen. In 1987, we started the first experiments under the inspiring direction of Prof. Rudi van den Hoofdakker and hundreds of patients have participated in various investigations ever since.