The exact cause of seasonal affective disorder (SAD) is unknown.

The symptoms of depression are thought to be linked to the shorter days of the year during the winter months, when the body has less exposure to sunlight.

Experts across the world have been researching SAD since the beginning of the 1980s and have developed several theories about what may trigger the condition. These are discussed below.

Effect on the hypothalamus

The amount of sunlight you receive affects some of your body's hormones and the chemicals in the brain. However, the effect that sunlight has on the brain isn't fully understood.

One theory is that light stimulates a part of the brain called the hypothalamus, which controls your mood, sleep and appetite. These can affect how you feel.

In people with SAD, lack of sunlight and a problem with certain chemicals in the brain prevents the hypothalamus working properly. The lack of light is thought to:

• affect the production of the hormone melatonin
• affect the production of the hormone serotonin
• disturb your circadian rhythms (the body's internal clock that regulates several biological processes)

Melatonin

Melatonin is a hormone that affects sleep. It's produced by a small gland in your brain called the pineal gland. When it's dark, the pineal gland produces melatonin to make you feel sleepy.

When it's light, the retina (the light-sensitive layer of cells at the back of your eye) converts the light into an electric impulse that travels to the hypothalamus. The hypothalamus sends a message to several glands in the body, including the pineal gland, which produces less melatonin.

During the winter months some people produce higher than normal levels of melatonin, resulting in SAD symptoms such as sleepiness and low energy levels.

Serotonin

Serotonin is another hormone that affects mood, appetite and sleep. It's also a neurotransmitter, which means that it transmits messages between nerve cells. When there are fewer hours of sunlight, less serotonin is produced.

If you have SAD, your serotonin levels may be lower than average during the winter. This may cause the messages between nerve cells to not be transmitted effectively, leading to the symptoms of SAD, such as feeling down.

Circadian rhythm

Your body uses light sources such as the sun to time various important biological functions and processes, including your mood, sleep, appetite, digestion and energy levels.

This regulation of the body's functions is known as a circadian rhythm, which usually occurs over a 24-hour period. However, a lack of sunlight can sometimes disrupt the body's internal clock, leading to SAD symptoms such as depression and low energy levels.

Other causes

Many different causes and contributing factors are associated with depression, some of which may also contribute to SAD. Possibilities include:

• genetic and family factors – a family history of depression could mean that you inherit particular genes that make depression more likely
• adverse childhood experiences – such as neglect or physical or sexual abuse
• personality – for example, you may be naturally quite anxious
• psychological factors – this relates to how the brain functions, such as your emotions and the way you think about things
• social factors – your relationships with other people, for example, whether or not you have close friends