The Prevalence Of Seasonal Affective Disorder: Where Is It Most Common?

What is seasonal affective disorder?

Seasonal affective disorder (SAD) is a major depressive disorder related to seasonal changes. Most people with SAD will have symptoms that begin in the autumn and winter months and resolve in spring or early summer. This symptom pattern is called winter-pattern SAD. 

Less often, people experience summer-pattern SAD, where their symptoms begin in spring or early summer and resolve in autumn or winter. 

Common symptoms of seasonal affective disorder

As SAD is a type of major depressive disorder, symptoms are similar to depression. If you have winter-pattern SAD, symptoms typically include: 

• Social withdrawal

• Loss of interest in most activities

• Low mood

• Fatigue

• Sleeping more than usual

• Carbohydrate cravings

If you have summer-pattern SAD, you may encounter: 

• Difficulty sleeping

• Anxiety

• Agitation

• Poor appetite

What causes seasonal affective disorder?

Scientists are not entirely sure what causes SAD, but research indicates several factors may be behind the condition:

• Low serotonin

• High melatonin

• Low vitamin D 

If you have seasonal affective disorder, you may have reduced levels of the neurotransmitter serotonin. Serotonin helps with mood regulation, so reduced levels can lead to depression symptoms. 

Other studies show that people with SAD may overproduce the hormone melatonin, which regulates sleep-wake cycles. Your melatonin levels rise when light decreases in the evenings, letting your body know it’s time to sleep. 

The shorter and darker days in winter may cause people with SAD to produce too much melatonin in the day, leading to them feeling lethargic. 

The combination of reduced serotonin and increased melatonin levels may disrupt circadian rhythms in people with SAD. Your circadian rhythm is your body’s internal “clock” responding to seasonal daylight cycles.

Serotonin and melatonin help maintain these circadian rhythms, so changes in these hormone levels can impact these rhythms. This can make it difficult to adjust to seasonal day-length changes. These disruptions can cause sleep and mood issues. 

Additionally, low vitamin D levels in winter may play a role in SAD. Researchers believe vitamin D supports serotonin activity, and your body produces it when exposed to sunlight. With less daylight in winter, people with SAD may have reduced vitamin D levels, hindering serotonin production. 

There are fewer identified factors for causes of summer pattern SAD, but increased sunlight exposure in summer may cause issues with melatonin production, leading to insomnia and irritability. Increased pollen counts in summer may also affect mood. 

SAD is more likely to occur in some groups than others

 SAD is more common in:

• Females

• People living far from the equator

• People with another type of depressive disorder

• People with other mental health conditions, such as anxiety disorders

• People who have relatives with SAD

• People who have relatives with other mental illnesses

Where is it most common?

SAD was first identified in the 1980s by Dr. Norman Rosenthal after he moved from South Africa to New York. He noticed he felt sluggish and less productive in winter, but he felt back to his usual self once spring arrived. 

He and his colleagues described seasonal affective disorder as a clinical condition after studying a group of people in Maryland. 

SAD has now been described in many other parts of the world. Scientists believe that reduced light exposure in winter plays a significant role in seasonal affective disorder.

In Rosenthal's early research,¹ light therapy eased depressive symptoms in patients. These findings suggested that populations living far from the equator were more prone to SAD due to little light in winter. 

SAD prevalence research in the US supported this hypothesis. For example, 1% of people in Florida experience SAD, while 9% of people in Alaska experience the condition. 

Some places with the highest prevalences of winter SAD include:

• Maryland (4.3%)

• New York (4.7%)

• Calvi, Italy, (5.0%)

• Napoli, Italy (6.3%)

• Montgomery County (6.3%)

• Nashua (9.7%)

Places with a high prevalence of summer SAD include:

• Capri, Italy (10%)

• Australia (9.1%)

Genetic and psychological factors may also play a role

The patterns of higher prevalence with increased distance from the equator do not appear to hold in some cases. Research² has shown no seasonal differences in depression symptoms in Iceland, far north of the equator. 

In other studies,³ SAD was unexpectedly low given Iceland's high latitude. These patterns in Iceland have led researchers to theorize that there could be a unique genetic feature in Icelandic populations that makes them less prone to developing winter depressive symptoms.

Research on Canadian populations of Icelandic descent showed they had lower SAD prevalence than the population of the east coast of the US. 

In Tromsø, a town in Norway at a latitude of 69ºN, winter depression rates have also been lower than expected. The town encounters extreme light variations throughout the year, with a polar night from November to January when the sun doesn't rise. 

Yet, winter depression rates are lower than expected, given its low light exposure. 

Recent research⁴ suggests that a positive winter mindset may contribute to these patterns in Norway. A positive attitude was strongly associated with good well-being in winter. 

However, more research into positive winter mindsets is needed to establish a causal relationship between seasonal perspectives and well-being. 

Treatment options for seasonal affective disorder

There are various treatment options for SAD, and you can use them in combination. Light therapy can be effective, as it's believed to mimic natural sunlight and assist your serotonin production.

Antidepressants can regulate serotonin, and studies⁵ have shown them to be as effective as light therapy for relieving SAD symptoms. Psychotherapy⁶ is another highly effective treatment. 

If you think you may be experiencing seasonal affective disorder, talk to your healthcare provider to discuss treatment options. 

The lowdown

Seasonal affective disorder is a type of depressive disorder that is recurrent and related to seasonal changes. It is more common in certain groups, and seasonal light exposure may play a significant role in the condition. 

Some research suggests SAD is more common in places further from the equator. While this prevalence pattern occurs in many cases, there are outliers, so genetic and psychological factors may contribute to the differences. 

If you’re concerned about seasonal affective disorder, speak to your doctor.