Causes of seasonal affective disorder

What causes seasonal affective disorder (SAD)?

There isn’t one confirmed cause of seasonal affective disorder (SAD). Researchers suggest it develops through a combination of biological, environmental and genetic factors. For winter-onset SAD, research has particularly focused on how seasonal changes in daylight may affect the body’s internal clock, or circadian rhythm, and biological processes involved in mood and sleep.

When the seasons change, the amount and timing of daylight changes too. This can influence the circadian rhythm, which helps regulate the sleep-wake cycle, as well as biological processes involving hormones and neurotransmitters such as melatonin and serotonin. Exactly how these changes interact, and why they may contribute to SAD for some people but not others, isn’t fully understood.

Most research into the causes of SAD has focused on winter-onset SAD because it’s more common. There is much less evidence about summer-onset SAD.

How can daylight affect SAD?

One of the leading explanations for winter SAD involves how some people’s bodies and brains respond to seasonal changes in daylight.

Daylight does more than help us see. Light reaching the eyes provides information to the brain about the time of day. This helps coordinate systems involved in the sleep-wake cycle, alertness, activity and other daily processes.

When the days become shorter and there are fewer hours of daylight, particularly from autumn through the winter months, this environmental signal changes. For someone who is sensitive to these changes, the shift may affect the timing and regulation of their body clock and other biological processes linked to mood.

This may help explain why SAD follows a seasonal pattern. It isn’t simply that winter is darker or that someone dislikes cold weather. Rather, the changing pattern of light and darkness may interact with the way an individual’s brain and body respond to the seasons.

How does light affect the hypothalamus and the brain?

Light helps the brain tell whether it is day or night and keeps the body’s internal clock in sync with the outside world. Light-sensitive cells in the eyes send signals to an area of the hypothalamus called the suprachiasmatic nucleus (SCN), which acts as the brain’s main body clock.

The SCN helps regulate daily processes such as sleep and wakefulness and the timing of melatonin production. When daylight changes with the seasons, the light signals reaching the SCN change too. In winter, shorter days may affect these rhythms in people who are particularly sensitive to seasonal changes in light.

This is one possible explanation for how seasonal changes in daylight may contribute to SAD, although the biological processes involved are complex and not yet fully understood.

How can SAD disrupt your circadian rhythm or body clock?

In some people with winter SAD, seasonal changes in daylight may affect the timing of the body’s internal clock, making it less well aligned with the natural day–night cycle. This may affect sleep, wakefulness, energy and mood.

Your circadian rhythm is your body’s natural 24-hour clock, which helps regulate daily patterns such as when you feel awake or sleepy. Daylight helps keep this clock in sync with the outside world, with light information reaching the brain through the eyes.

This is sometimes described as the body clock being “out of sync” with the environment. However, circadian rhythm changes are unlikely to explain SAD on their own, as people may differ in their sensitivity to seasonal changes in light.

What role does melatonin play in seasonal affective disorder?

Melatonin may play a role in seasonal affective disorder (SAD) by influencing the body’s sleep-wake cycle. The body normally produces more melatonin during the biological night, and light exposure suppresses its production.

Research has found seasonal changes in melatonin production and timing in some people with SAD.

Shorter winter days and changes in the pattern of light and darkness may therefore affect when melatonin is produced, potentially disrupting daily rhythms and sleep. However, findings are mixed, and changes in melatonin are thought to be part of a wider set of biological processes rather than a single proven cause of SAD.

What role does serotonin play in seasonal affective disorder?

Serotonin may play a role in seasonal affective disorder, particularly winter-onset SAD. Research suggests that seasonal changes in daylight may affect serotonin-related processes in the brain, although findings are mixed.

This does not mean SAD is simply caused by “low serotonin”. A 2026 meta-analysis found no overall seasonal change in serotonin transporter availability in healthy people, highlighting the complexity of the evidence. Serotonin may interact with daylight, the body clock and other biological factors in SAD, but exactly how these processes contribute remains unclear.

Do some people need more light than others?

People may differ in how their eyes and brain respond to light. Specialised light-sensitive cells in the retina, including melanopsin-containing retinal cells, send information about light to areas of the brain involved in regulating circadian rhythms and other biological processes. Differences in this pathway may contribute to vulnerability to seasonal mood changes.

This may help explain why seasonal changes in daylight affect some people more than others. However, researchers do not yet know exactly how differences in light sensitivity contribute to SAD, or whether they are a cause or consequence of seasonal mood changes.

Is vitamin D deficiency linked to seasonal affective disorder?

Research has investigated whether vitamin D deficiency may be linked to seasonal affective disorder (SAD), but the evidence is mixed and doesn’t show that low vitamin D is a direct cause of SAD.

The body produces vitamin D when skin is exposed to UVB radiation from sunlight, so levels can be lower in winter. However, a randomised controlled trial didn’t find a clear benefit from vitamin D supplementation for seasonal affective symptoms. This means vitamin D deficiency has not been established as a direct cause of SAD, and its role in seasonal changes in mood remains unclear.

What causes summer seasonal affective disorder?

SAD is most commonly associated with winter, but summer SAD (sometimes called summer-pattern SAD) does occur.

Much less is known about what causes summer-onset SAD. Researchers have proposed that longer daylight exposure and shorter nights could affect circadian timing and melatonin production in some people.

Environmental factors such as higher temperatures may also affect sleep, which could influence mood. However, there is much less evidence about the causes and biological mechanisms of summer-pattern SAD. These factors could help explain why seasonal depression can occur in summer as well as winter. However, they have not been studied as extensively or established as causes in the same way as the biological mechanisms proposed for winter-onset SAD.

Who is more likely to develop seasonal affective disorder?

Seasonal changes in daylight don’t affect everyone in the same way. Researchers have identified several risk factors associated with SAD, although having one of these factors does not mean that someone will develop the condition.

Do genes and family history increase your risk of SAD?

SAD appears to have a familial component. It’s more common among people who have a family history of SAD or other mental health conditions, suggesting that genetic factors may contribute to someone’s vulnerability.

This does not mean there is one specific gene that causes SAD. Several genetic factors may contribute to vulnerability to SAD, potentially through characteristics such as sensitivity to seasonal changes, mood and circadian rhythms. These genetic influences may interact with environmental factors such as changes in daylight.

Does age affect your risk of seasonal affective disorder?

SAD can occur at any age, but it often begins in young adulthood. It appears to be less common in later adulthood.

This is a risk pattern rather than an explanation for why SAD develops. Researchers don’t yet know exactly why age might influence vulnerability to seasonal depression.

Does where you live affect your risk of seasonal affective disorder?

Geography may also influence the likelihood of SAD.

Some studies have found that SAD is more common at greater distances from the equator, where seasonal changes in daylight hours tend to be larger. However, findings have not consistently shown a simple relationship between latitude and SAD. This provides further evidence that changing exposure to daylight may be relevant to winter SAD.

Location isn’t a straightforward cause, as rates don’t simply continue increasing the further north someone lives. This suggests that other factors, including individual vulnerability, climate and genetics, also matter.

Are women more likely to develop seasonal affective disorder?

SAD is reported more often in women than in men. Some population studies have found that winter SAD is around one-and-a-half to twice as common in women, although the size of the difference varies between studies and some research has not found a significant difference.

Researchers don’t yet know why women may be more vulnerable to SAD. Sex differences in biological responses to seasonal changes have been investigated, but there isn’t enough evidence to identify a specific biological mechanism as the cause.

Can stress or difficult life experiences cause seasonal affective disorder?

Not every episode of depression that happens at the same time each year is SAD. Difficult circumstances associated with a particular season or time of year can affect mental health, but SAD is specifically characterised by a recurring seasonal pattern. How psychological and environmental factors interact with the biological changes associated with the seasons is still being investigated.

Is SAD linked to depression and bipolar disorder?

SAD is a type of depression with a recurring seasonal pattern. Seasonal patterns can also occur in bipolar disorder, particularly bipolar II disorder.

Seasonal patterns can occur in both bipolar disorder and major depressive disorder, so the relationship isn’t simply that one condition causes the other. Instead, some people may have a greater underlying vulnerability to mood changes that follow a seasonal pattern.