As winter approaches, many people experience a dip in mood and energy, but for some, this seasonal shift can be debilitating. Seasonal affective disorder (SAD), a type of depression that follows a predictable seasonal pattern, affects around 2% of the UK population, while a further one in five experience milder 'winter blues'. Researchers are now uncovering the biological mechanisms behind these changes, offering new insights into treatment.
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 changes in human mood 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, revealing links between reduced daylight, circadian rhythm disruption, and mood.
Light is the key regulator of the body's internal clock, influencing hormone release, alertness, and mood. Without sufficient daylight, especially when combined with too much artificial light in the evenings, these rhythms can drift out of sync. Some individuals may be more sensitive to these shifts; for example, people with bipolar disorder often experience manic episodes in spring and depression in winter, suggesting heightened sensitivity to light cues.
Seasonal biology extends beyond mood. Researchers have found fluctuations in the expression of over 4,000 protein-coding genes in white blood cells and fat, as well as changes in blood cell composition across the year. Wyse and her colleagues analysed four years of UK Biobank sleep data from half a million people and found that in winter, people tended to sleep longer but had poorer sleep quality compared to summer. These findings will be presented at the British Sleep Society conference in Brighton next week.
Bright light therapy remains the gold standard for treating SAD, with timing and dosage critical. Natural sunlight is a powerful substitute for those unable to use a light box. Studies of Amish communities in the US, who spend more time outdoors and less time exposed to bright artificial light at night, show some of the lowest rates of SAD among Caucasian populations. Exposure to bright natural light early in the day helps synchronise the circadian clock and suppress melatonin, the hormone that can cause morning grogginess.
