Seasonal Affective Disorder and Bipolar Disorder: How Is It Linked?

Seasonal affective disorder (SAD) is a mood disorder characterized by depressive symptoms that present in response to seasonal weather and climate changes. It is a subtype of major depressive disorder and bipolar disorder.

The most common symptom of SAD is atypical depressive symptoms, including weight gain and an increased need for sleep. These symptoms are also often associated with bipolar disorder. Typical depressive symptoms include decreased sleep and loss of weight. 

What is seasonal affective disorder?

Seasonal affective disorder is also commonly known as seasonal depression or the “winter blues.” Approximately 5% of the US population¹ is diagnosed with SAD. 

The people most at risk of developing seasonal affective disorder are females, those between the ages of 18-30 years old, those who live far from the equator, and those with a family history of depression or SAD.

Typical symptoms of SAD include depression, increased need for sleep, increased appetite, and difficulty concentrating. 

These symptoms tend to last for around 40% of the year,¹ with full recovery during the remainder of the year. 

Symptoms of SAD often arise due to weather and seasonal changes. Typically, people who experience SAD have depressive symptoms in the fall or winter. 

Causes of seasonal affective disorder

It is thought that seasonal affective disorder is caused by a disruption to your circadian rhythm (biological clock), changes to melatonin production, and a decrease in available vitamin D. 

Increase in melatonin production

Melatonin is a hormone that is produced by the pineal gland in your brain and is then released into your bloodstream, causing you to feel tired. This should occur at nighttime, starting two hours before you sleep. 

Melatonin production is inhibited by sunlight, so during the day, your blood should contain very little melatonin. 

During the winter, when there is less sunlight during the day and the days become shorter, your brain may produce too much melatonin.² This causes you to feel fatigued during the day, interrupting your biological clock. Excessive tiredness is a common symptom of SAD. 

Decreased availability of vitamin D

The majority of the vitamin D absorbed by your body comes from sunlight. During the winter months, there is significantly less sunlight than in summer. This can cause a vitamin D deficiency. 

Vitamin D is required for the synthesis of a molecule called serotonin, which is critical for stabilizing your mood. If you do not have enough vitamin D to produce serotonin, your mood may be altered, resulting in the depressive symptoms of SAD.

What is bipolar disorder?

Bipolar disorder is a mood disorder in which behaviors fluctuate between two stages — manic and depressive. It is also known as manic-depressive disorder. 

Approximately 4.4% of the US population is diagnosed with bipolar disorder.³ This condition is more common in the US⁴ than in any other country, as the global average is only about 2%. If you have a family history of bipolar disorder or you have experienced stressful or traumatic events in your life, you are more at risk of developing bipolar disorder. 

Bipolar disorder typically develops during late adolescence or early adulthood and affects both men and women equally. 

If you have bipolar, you can experience manic episodes. These may cause increased energy, restlessness, difficulty sleeping, irritability, or feeling as if you do not need to sleep. 

Depressive symptoms of bipolar disorder include atypical depression or intense feelings of sadness, hopelessness, inability to complete simple tasks, increased need for sleep, or even suicidal thoughts.

Mania and depression can sometimes occur together. This is called a mixed episode. 

The main types of bipolar disorder 

There are three main types of bipolar disorder:

• Bipolar I

• Bipolar II

• Cyclothymic disorder

Bipolar I

A person with bipolar I cycles between depressive episodes that last at least two weeks at a time and manic episodes lasting at least seven days. You may not always experience depressive episodes, as you may only go through the manic phases of the disorder.

Sometimes, severe manic episodes require hospitalization. People with bipolar I may experience mixed episodes.

Bipolar II

Bipolar II differs from bipolar I as it involves episodes of hypomania rather than mania. Hypomanic episodes are shorter and less intense than full manic episodes. 

People with bipolar II also experience depressive episodes as well as mixed episodes.

Cyclothymic Disorder

Cyclothymic disorder — also called cyclothymia — is a mood disorder similar to both bipolar I and II as it involves periods of depressive episodes as well as hypomania. 

Cyclothymia mood swings are less severe than in bipolar I and II, and the episodes tend to be shorter. 

What causes bipolar disorder?

The exact cause of bipolar disorder is not well understood, but it is thought to be related to genetics, environmental factors, and brain structure. 

Depressive episodes of bipolar disorder may be caused by abnormal functioning of the brain circuits that transmit serotonin. This can result in there being insufficient amounts of serotonin in your nervous system, reducing your ability to regulate your mood effectively. 

What is the link between bipolar disorder and seasonal affective disorder?

Seasonal affective disorder is a subtype of bipolar disorder. Bipolar disorder can be influenced by the environment, causing seasonal patterns in bipolar symptoms. Approximately 15-22% of people diagnosed with bipolar disorder experience seasonal patterns in their symptoms. 

Research also suggests that 11-50% of people diagnosed with seasonal affective disorder⁵ may fit the criteria for a diagnosis of bipolar disorder.

If you have bipolar disorder with seasonal patterns⁶ (BD SP), you may tend to experience depressive episodes during the winter months — as is common with seasonal affective disorder — and manic episodes during the summer months. 

BD SP may have similar causes as seasonal affective disorder. The change in daylight hours and decrease in sunlight may play a role in the onset of depressive symptoms.⁶ 

Light therapy

Light therapy is a first-line treatment for seasonal affective disorder. High-intensity light is used to imitate morning sunlight to regulate your body’s circadian rhythm and temporarily inhibit melatonin production. 

Light therapy has been shown to be effective⁵ for treating bipolar disorder with seasonal patterns. 

If you experience symptoms of bipolar disorder with seasonal patterns, you should discuss with your doctor whether light therapy is right for you before starting any treatment. This is because light therapy may trigger⁶ a switch from a depressive episode to a manic or hypomanic episode if you have bipolar disorder.

Seasonal affective disorder vs. bipolar disorder

The table below will help you to understand the differences and similarities between seasonal affective disorder and bipolar disorder:

The lowdown

Seasonal affective disorder involves the onset of depressive symptoms in response to changes in the weather and seasons. Bipolar disorder is a mood disorder where you shift between episodes of mania or hypomania and depression.

If you experience symptoms of seasonal affective disorder or bipolar disorder with seasonal patterns, you should discuss this with your doctor to determine what treatment is right for you.