Shining a light Scientists try to understand biology behind seasonal affective disorder

For some, the darkening days of autumn bring more than the annual ritual of reviving woolly jumpers and turning on the central heating.

- Linda Geddes

As the evenings close in and the mornings grow murky, energy ebbs and a heavy sadness settles in.

Although seasonal affective disorder (Sad) was only formally recognised by psychiatrists in the 1980s, the link between the seasons, mood and vitality has long been observed.

The Yellow Emperor's Classic of Medicine - a Chinese text from roughly 300BC - described how the seasons affect all living things. It advised that in winter, one should “retire early and get up with the sunrise”, keeping “desires and mental activity quiet and subdued, as if keeping a happy secret”.

Today, scientists are rediscovering how closely attuned our biology is to the seasons. According to Dr Cathy Wyse, a chancellor's research fellow at the University of Edinburgh, the biggest advance over the past decade “has been the realisation that seasonal changes in the human mood are probably endogenous - that they are part of our physiology”.

Large-scale resources such as the UK Biobank have transformed this area of research, allowing scientists to track seasonal patterns across hundreds of thousands of people over many years - something that was previously impossible.