Seasonal Affective Disorder Statistics: Symptoms, Treatments, Types, And More

The prevalence of SAD: how common is it?

Seasonal affective disorder (SAD) is a variant of major depressive disorder (MDD) that involves returning episodes of depression around the same time each year. These depressive episodes often present most strongly in the winter and go away in the summer. 

However, individuals can experience these depressive periods in any season and still be diagnosed (major depression with a seasonal pattern) as long as those periods of depression are consistently reduced during other seasons. 

Reports of the actual prevalence of SAD can vary immensely from study to study.

What does the research show?

A 2018 longitudinal cohort study¹ based in Zurich suggests that MDD with a seasonal pattern (meeting the specific criteria of two seasonal depressive episodes over two years) affects about 3.44% of the general population. 

Experiencing one depressive episode in winter is much more common - about 9.96% of the Zurich population. However, a single winter-time depressive episode does not meet the SAD diagnostic requirement. 

Further studies² from America have suggested that seasonal affective disorder affects about 4-6% of the general population. 

When further examining the prevalence of the seasonal affective disorder, it should be noted that certain populations are more likely to be affected than others. Similar to other depressive disorders, SAD tends to be more prevalent in women than men. 

The magnitude of difference, however, varies between reports. Some experts suggest that seasonal affective disorder is 1.5 times³ as prevalent in women than men. The National Institute of Mental Health⁴ states that women are four times more likely to develop SAD than men. 

SAD is generally first diagnosed between the ages of 18 and 30, but SAD can and does affect any age group. Family history can also play a role in the prevalence of SAD. 

Those with a family history of depression and suicidality have a higher prevalence of long-term seasonal affective disorder than those with no history. 

Individuals with certain disorders can also present a higher likelihood of developing SAD. Those with delayed sleep phase syndrome⁵ are about 3.3 times more likely to have the seasonal affective disorder. 

Researchers have even reported⁶ that seasonal affective disorder is more common in individuals with severe visual impairments. This may indicate a link between SAD symptoms and a lack of retinal light input.

Some individuals may experience symptoms of feeling sad during the winter months but do not meet the criteria to be diagnosed with seasonal affective disorder. These individuals are likely experiencing what is commonly referred to as the “winter blues,” 

This condition is similar to seasonal affective disorder, though it lacks the severity or additional symptoms to be diagnosed. Experts report²+ the prevalence of winter blues is about 10-20% of the general population - much higher than the prevalence of SAD. 

However, the real number may be much higher, as winter blues is not a diagnosed condition, and individuals may not report symptoms. Like SAD itself, winter blues tend to be more prevalent⁷ in women than men. 

Why do some countries have higher SAD rates than others?

Seasonal affective disorder, especially in cases with depressive episodes primarily in the winter, is proposed to be caused by short days and low sunlight exposure. 

Decreased UV exposure may cause vitamin D deficiency, changes in serotonin neurotransmission, and increased melatonin synthesis, which may contribute to depressive symptoms. 

It then seems logical that areas further from the equator, with shorter days in the winter, harsher weather, and more intense changes in seasons, would have a higher prevalence of SAD. 

This theory has been evidenced by disproportionately high rates of seasonal affective disorder in Alaska⁸ over both age and gender. In Alaska, it has been estimated that about 10% of the population experiences seasonal affective disorder. 

On the other hand, in Australia, which has rather mild winters and exists relatively close to the equator, SAD is rare. Some estimates⁹ put its prevalence at 0.3% of the general population.

Contrary to this theory, however, SAD is rare in some populations well north of the equator, such as Iceland and Finland. 

These populations experience much shorter days in the winter, but they may have adapted over the centuries to handle this change better. They may also demonstrate behavioral changes that facilitate a lower likelihood of developing SAD. 

A 2021 exploratory study¹⁰ found that seasonal affective disorder was more common in college students who moved from areas with little seasonal variability to areas with large seasonal variability.  

This finding further indicates that individuals in areas with significant environmental changes from season to season (such as those far from the equator) may be better adapted to dealing with seasonal affective disorder.

Common SAD symptoms and statistics

While seasonal affective disorder is relatively common, its presentation can vary widely across different people. Symptoms can differ and may depend on many external factors; however, some common symptoms include:

Low mood

Individuals with SAD commonly experience low mood as a primary symptom. 

Sleep disturbances

As with other forms of major depressive disorder, individuals often experience disturbances to their sleep. In seasonal affective disorder¹¹, the majority of individuals (63%) experience an increase in total normal sleep (hypersomnia), with a lower proportion experiencing decreases in sleep (37%).

It is also important to note that 65% of individuals with SAD experience poor sleep quality and excessive daytime drowsiness.

Social withdrawal

Individuals with seasonal affective disorder often withdraw from their friends and family. This may be prompted by the changes in weather or by symptoms of the condition itself. This withdrawal occurs in nearly all (98%) of SAD cases.

Common SAD treatment types and statistics

Seasonal affective disorder can have a large, negative effect on some individuals' lives. There are many different treatments for seasonal affective disorder, all with varied effectiveness and suitability for certain people. These treatments include:

Light therapy

Light therapy is one of the most common treatments for SAD. It involves using light-emitting boxes, often in the morning, for about 20-40 minutes per day. This treatment aims to supplement sunlight exposure during the winter months. 

Light therapy provides relief from many SADs symptoms, and there are a few proposed theories why. One of these is the shifting of users’ sleep-wake cycle. 

Many individuals with SAD have altered sleep-wake cycles¹², which can produce some of the symptoms above. Morning light treatment can replace natural sunlight and trigger a set of internal reactions - it can effectively “wake up” an individual and reset their sleep cycle. 

Others propose that lightboxes work through the vitamin-D pathway - they believe light exposure facilitates increased vitamin D synthesis. This increased vitamin D, in turn, may reduce depressive symptoms. 

Vitamin D is required for serotonin synthesis and increases serotonin levels. Low serotonin neurotransmission is common in individuals with SAD¹³, so an increase in serotonin levels may also reduce symptoms.

Light therapy has been used extensively in SAD treatment with varied but mostly positive results. 

A placebo-controlled light treatment study¹⁴ suggests that 61% of individuals demonstrate at least a 50% reduction in depressive symptoms with morning light therapy (assessed by SIGH-SAD measure) compared to 32% with a placebo treatment. 

Another exploratory study¹⁵ found that 70% of patients undergoing lightbox treatment experienced a 50% reduction in depressive scores. That’s similar to peoples’ antidepressant medication response. 

However, when post-treatment scores were examined, light therapy was more effective than antidepressants, with 50% and 25% of patients experiencing significant symptom reductions. These results were, however, not statistically significant. 

Antidepressants

As with many other depressive disorders, SAD may be triggered by a dysregulation in serotonin neurotransmission¹⁵. Thus, antidepressant medications that balance serotonin neurotransmission are viable options for many people with SAD. 

These medications are usually used within the depressive season and are tapered off in remitting seasons. Selective Serotonin Reuptake Inhibitors (SSRIs) are the first line of pharmacological treatment for SAD. A systematic review¹⁶ considering fluoxetine (an SSRI) suggests it has higher efficacy than a placebo control. 

Similar numbers (approximately 67%) of individuals responded positively to fluoxetine and light therapy within a group of SAD-diagnosed individuals, demonstrating the similar efficacy of both treatments. Patients who were given fluoxetine also only had a 54% remission rate.

An additional study¹⁷ considering sertraline (another SSRI) found that 62% of SAD-diagnosed individuals responded to treatment while only 46% responded to placebo. 

Antidepressant medications also have proven efficacy in preventing depressive episode recurrence. 

One study¹⁸ investigating Bupropion XL found that starting bupropion early can help prevent future SAD episodes. 

Cognitive behavioral therapy

Cognitive behavioral therapy (CBT) is a form of talk therapy that has been extensively used to treat patients with many variants of major depressive disorder. It follows the general principles that symptoms of depression originate from learned, unhelpful behaviors and underlying negative thoughts. 

CBT aims to make these thoughts and behaviors more functional and, in turn, reduce symptomatology. Many studies have indicated CBT’s efficacy in treating SAD. 

One study suggests¹⁹ that CBT, either alone or with light therapy, significantly reduces SAD symptoms. A CBT and light therapy combination were particularly successful in producing post-treatment reductions in depressive symptoms. 

This study also noted that no CBT-treated individuals with or without supplementary light therapy experienced relapses in SAD symptoms. However, 60% of individuals who only undertook light treatment did relapse. 

Additionally, CBT effectively prevents depressive episode recurrence²⁰ in SAD even two winters after therapy. 

Only 27.3% of CBT-treated patients experienced recurrent depressive episodes, while 45.6% of light therapy-treated individuals had a SAD recurrence. Individuals who were CBT-treated also experienced less severe symptoms.

While the above treatments are the most common ways to treat SAD, they are not the only options. Exercise, changes in diet, and vitamin D supplements have also been used to treat SAD.

What do the statistics say about SAD?

Statistics demonstrate that SAD is a relatively common form of MDD and tends to have a higher prevalence in certain populations, including women, those with delayed sleep phase disorder, and those with a family history of depression.

Doctors or counselors for such individuals must be aware of their patients’ heightened SAD risk and make the appropriate preparations to reduce the likelihood of this disorder developing.

The statistics also demonstrate that, despite the logical hypothesis that individuals further from the equator should have a higher prevalence of SAD, this trend is not true in all cases. Some individuals in northern populations actually have a lower prevalence of SAD, possibly due to genetic or behavioral adaptations.

Research has also demonstrated a vast number of diverse and variably prevalent symptoms are associated with this disorder. The most common symptoms are low mood (particularly sadness) and social withdrawal.

These symptoms and others have many possible treatments, all of which have varied rates of use and effectiveness. The efficacy of such treatments must be assessed before they are tried.

The lowdown

Seasonal affective disorder is a prevalent disorder among the general population. SAD’s prevalence varies across different populations and may be affected by factors including comorbid disorders, gender, and age.

Despite all this variability, many common SAD symptoms can impact an individual's well-being. If you or someone you know is experiencing a seasonal affective disorder, do not be afraid to seek help in the form of the treatments above.