The cause of seasonal affective disorder (SAD) is not fully understood but is thought to be linked to reduced exposure to light during the shorter days of the winter months.

Hypothalamus

Research has shown that bright light makes a difference to brain chemistry, but it is not clear exactly what the effect is. One theory is that light stimulates a part of the brain called the hypothalamus, which controls mood, appetite and sleep. In people with SAD, lack of light and a problem with certain brain chemicals prevents the hypothalamus from working properly.

Some of the chemicals and hormones that can affect SAD are outlined below.

Melatonin

Melatonin is a hormone which affects the way we sleep. When it is dark a small part of the brain, known as the pineal gland, starts to produce melatonin to help us sleep. When it is light we produce less melatonin, helping us to wake up.

People with SAD produce much higher levels of melatonin in winter than normal. Research has shown that in the summer months the melatonin levels of people with SAD tend to return to normal. Using light therapy and medication to affect melatonin levels will not completely cure SAD, which suggests that the condition is caused by a combination of factors.

Serotonin

This is a chemical found naturally in the brain which can affect your mood, appetite and sleep. If you have SAD, you will usually have lower serotonin levels than average during the winter months. Serotonin is a neurotransmitter, which means that it helps to transmit messages from the nerve cells to the brain. It is thought that the serotonin in people with SAD may not be able to transmit messages effectively. Serotonin is affected by sunlight. Reduced sunlight means that less serotonin is produced.

Circadian rhythm

Your circadian rhythm is a psychological process that helps to regulate your body's internal clock, letting you know when you should sleep and when you should wake. A reduced level of sunlight can sometimes disturb your circadian rhythm. Disruption to your sleeping and waking pattern has often been linked to feelings of depression.

Genetic cause

Research suggests that SAD may be passed on genetically. If you have a close family relative (mother, father, brother, sister) with SAD you have a one in seven chance of inheriting the condition.