Researchers are uncovering the biological mechanisms behind seasonal affective disorder (SAD), a condition that affects mood and energy levels as days grow shorter. While SAD was only formally recognised in the 1980s, ancient texts such as the Yellow Emperor's Classic of Medicine (circa 300 BC) advised retiring early and rising with the sun during winter to maintain mental calm.
Dr Cathy Wyse, a chancellor's research fellow at the University of Edinburgh, notes that the biggest advance in the past decade has been the realisation that seasonal mood changes are likely endogenous—part of our physiology. Large-scale studies like the UK Biobank have allowed scientists to track seasonal patterns across hundreds of thousands of people over many years.
SAD is a subtype of major depression or bipolar disorder, with symptoms typically beginning in autumn or winter and lifting in spring. Alongside typical depression signs, sufferers often sleep longer, feel lethargic, and crave carbohydrates. Reduced daylight is thought to be the key trigger, as light regulates the circadian clock and hormone release.
Wyse and her colleagues are studying seasonal effects in bipolar disorder, where manic episodes peak in spring and depression is more common in darker months. They suspect that people with bipolar disorder may be highly sensitive to light and seasonal cues. Even without clinical depression, about one in five people in the UK experience milder 'winter blues'.
Beyond mood, researchers have found seasonal fluctuations in the expression of over 4,000 protein-coding genes and changes in blood cell composition. Wyse's analysis of UK Biobank sleep data from half a million people found that in winter, people sleep longer but have poorer sleep quality. Bright light therapy remains the gold standard treatment for SAD, with natural sunlight as a powerful substitute.
