Bailey Earls|12/27/2022|4 min read

What Is Seasonal Affective Disorder?

The Circadian Rhythm and Light Therapy


As the days get shorter and the weather colder, some people may begin feeling drowsy, less energetic and hopeless. But this isn’t simply a case of the “winter blues”. In fact, it could be something that should be taken more seriously.

If you notice a change in your habits or behavior during the change of seasons, you may be experiencing Seasonal Affective Disorder (SAD).

Here you can learn more about seasonal depression and the steps you can take to manage your mental health throughout the year.

Seasonal Affective Disorder (SAD)

According to the Diagnostic Manual of Mental Disorders (DSM-5), Seasonal Affective Disorder  is not a disorder of its own but a form of depression. It is characterized by the occurrence of seasonal patterns. 

SAD typically begins and ends at the same time every year. Most cases occur during late fall or early winter and subside during late spring and early summer. The most difficult months for those with SAD are January and February. 

If you do experience some seasonal depression, don’t worry, you’re not alone. Approximately 10 million Americans are affected by SAD and 10 to 20% have mild symptoms.

Cases of SAD are more common in those living far north or south of the equator. This is due to the shorter daylight hours of winter. People living in New England or Alaska tend to be more at risk than people living in Florida or Hawaii. 


SAD usually occurs in adulthood and affects women more often than men. The risk increases with age; rarely affecting people under the age of 20. 

There’s an assumption that SAD only occurs during the winter months, and while it is more common, there are two kinds of seasonal depression. The second one is called Spring-onset or “summer depression”.

During Spring-onset, symptoms begin in the late spring to early summer and subside during late fall to early winter. In either case, symptoms may start off minimal and increase as the season progresses. Symptoms can last up to 4 to 5 months each year. 

Symptoms of SAD

The symptoms of SAD are not always the same, they exist on a spectrum. From a change in sleep habits to thoughts of self-harm and suicide. While these symptoms do not occur year round, they should still be taken seriously. 

Fall-onset symptoms

  • Increased appetite 

  • Weight gain

  • Tiredness or low energy

  • Oversleeping

  • Loss of interest in activities once enjoyed

Spring-onset symptoms

  • Lack of appetite

  • Weight loss

  • Increased irritability

  • Agitation or anxiety

  • Trouble sleeping (insomnia)

Causes of SAD

Scientists have not yet found a specific cause of seasonal affective disorder. But research has shown some factors that may have an impact. The lack of sunlight plays a big role in the amount of serotonin and melatonin created in the brain. Thus, affecting the functions of the hypothalamus. Serotonin is a neurotransmitter that affects mood and other biological functions. Serotonin levels are more likely to decrease due to the lack of sunlight. This can trigger depression-like symptoms. 

Reactionally, melatonin is a hormone created in response to darkness. It is how the body regulates its sleep-wake cycle. Due to the lack of sunlight, melatonin levels increase, changing the body’s sleep and mood patterns. 

Both serotonin and melatonin play an important role in your biological clock, also known as the Circadian Rhythm. With your body’s internal clock disrupted, feelings of depression are more likely to occur. Therefore causing complications in daily life, such as substance abuse, social withdrawal and lack of productivity. 

How does the Circadian Rhythm work?

The Circadian Rhythm is a physical, mental and behavioral change throughout a 24-hour day. The natural process is a response to the environment; such as light and dark, affecting most living things. Not all living things follow the same cycle, for instance, nocturnal animals sleep during the day and awake at night. 

A biological clock is an organism’s natural timing device. This device regulates the sleep-wake cycle, hormonal activity, eating and digestion. This is done through body temperature, blood pressure and natural production of hormones and chemicals.

Light Therapy treatment

According to the American Psychiatric Association, light therapy involves sitting in front of a light box for 20-30 minutes a day (may increase depending on the individual's symptoms), typically in the morning, during the fall and winter months.

A light therapy box emits a bright light while filtering out harmful ultraviolet (UV) rays. Most people see improvement in their symptoms after two weeks of use. Light therapy is not a cure for seasonal depression but helps maintain its symptoms. 

Unlike traditional light boxes, dawn simulators come on gradually. The gradual light increase exerts a waking effect that mimics that of a natural rising sun. It is considered to be a more natural effect on the circadian cycle. 

Other treatments

There are other treatments for SAD including cognitive behavioral therapy (CBT), Vitamin D and medications. The medications are typically antidepressants or selective serotonin reuptake inhibitors (SSRIs). These include: 

  • Citalopram

  • Sertraline

  • Escitalopram

  • Fluoxetine 

  • Paroxetine 

Vitamin D is believed to promote serotonin activity. A deficit in vitamin D may inhibit the body's production. Vitamin D can be consumed in diet but the body also produces vitamin D when skin is exposed to sunlight. The lack of light in the winter decreases vitamin D production. 

Seasonal changes and Bipolar Disorder

People who experience bipolar disorder have an increased risk of SAD. Bipolar disorder is a mental health condition that causes mood swings; where a person shifts between episodes of mania or hypomania and depression. Similar to SAD’s depressive symptoms, episodes of mania may be linked to a specific season in some people with bipolar disorder. 

For example, spring and summer can bring on symptoms of mania or hypomania with an increase in anxiety and agitation. Fall and winter can bring on feelings of depression. 

It’s important to note that bipolar disorder and seasonal affective disorder are not the same thing. Bipolar disorder occurs year round and SAD is only seasonal. Symptoms of SAD are affected by the change of season while bipolar episodes have multiple triggers. 

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