Seasonal affective disorder (SAD)

Seasonal affective disorder (SAD) is a type of depression that affects people in winter months, hence why it is also sometimes known as “winter” or “seasonal” depression. Its causes aren’t fully understood, but it is generally thought to occur at the time of year when a lack of sunlight disrupts activity in a part of the brain called the hypothalamus.

This can have three main effects: decreased production of the mood-boosting hormone serotonin, increased production of the sleep-inducing hormone melatonin and disruption of circadian rhythms – the body’s internal clocks. As a result, along with low mood and feeling depressed, people showing symptoms of seasonal affective disorder may be irritable, have disrupted sleep patterns that last longer than normal, exhibit low energy levels and crave carbohydrates.

Because of its link with lack of sunlight, one of the most common treatments for SAD is light therapy. This can take the form of regular use of a light box that produces bright, blue-enriched light. Increased exposure to natural sunlight can help too – indeed, exposure to sunlight soon after waking has been found to be as effective for general depression as antidepressant drugs.

Some research casts doubt on the concept of SAD, finding that seasonality isn’t a major factor for most people with depression. What’s more, countries at high latitude don’t necessarily have high levels of seasonal affective disorder.

That is the case in Norway, for example, even among people who live within the Arctic Circle, where the sun may not rise for months in winter. However, this could be partly explained by people’s attitude to winter there. A positive mindset is known to improve people’s health and well-being, and research suggests that this may be how Norwegians stave off negative mental effects from winter.