Have you considered clinical trials for Seasonal affective disorder (SAD)?

We make it easy for you to participate in a clinical trial for Seasonal affective disorder (SAD), and get access to the latest treatments not yet widely available - and be a part of finding a cure.

What is seasonal depression?

Seasonal depression, also known as seasonal affective disorder (SAD), is a mood disorder characterized by depressive episodes with a particular seasonal pattern. 

Symptoms occur during a particular season each year, then disappear when the season ends.

The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) classifies seasonal depression as a major depressive disorder with a seasonal pattern.  

Who gets seasonal depression?

Seasonal depression affects about 5% of people in the United States. It is particularly common in women and young adults. 


Women are four times² more likely to develop SAD than men. Experts suggest that this may be related to hormonal changes³ that occur due to inactivity and lack of physical activity. These changes result in a cascade of other hormonal changes associated with seasonal depression. 

Young adults

It is thought that young adults are more reactive to small changes in their environment³, such as the amount of sunlight they are exposed to. This may explain why the average age of SAD onset is 23 years old² and why it is so much more common in people aged between 18 and 30¹.

Types of seasonal depression

Seasonal affective disorder can occur in either summer or winter.

The most common type is winter-onset seasonal depression. Symptoms arise in the autumn, either late September and October in the Northern Hemisphere, or March and April in the Southern Hemisphere, and occur throughout the winter.

Although the symptoms may be mild at first, they can worsen throughout the winter, especially if left untreated. A hallmark of SAD is that symptoms almost completely disappear during the spring and summer, then recur the following autumn and winter. 

People who live far away from the equator in areas with fewer hours of sunlight are more likely to experience winter-onset seasonal depression.

Summer-onset seasonal depression follows the opposite seasonal pattern. People who live closer to the equator may be more likely to suffer from summer-onset seasonal depression, with symptoms starting to show in spring.

Symptoms of seasonal depression

Symptoms of SAD vary widely, but it is typical for symptoms to last for approximately 40% of the year⁴. It's important to be aware of the possible symptoms, so you know when to seek help and what to expect in future seasons when symptoms recur. 

Since seasonal depression is a subtype of major depressive disorder, it shares key symptoms with this condition. 

The DSM-5 criteria for a major depressive episode include having five or more of the following:

  • Loss of interest in the everyday activities that you once enjoyed

  • Depressed mood or irritability 

  • Unintentional weight loss or weight gain 

  • Sleep problems, including difficulty sleeping (insomnia) or excessive sleeping (hypersomnia) 

  • Feeling guilty or worthless 

  • Low energy levels and fatigue 

  • Difficulty concentrating and being indecisive 

  • Recurrent thoughts of death or suicide 

To be diagnosed with SAD, you need to meet the criteria for depression that begins and ends during a specific season every year, with a full recovery during other seasons, for two consecutive years.

SAD symptoms tend to be mild at first, often involving headaches, poor concentration, increased irritability, or fatigue, which can lead to other symptoms. 

In addition to the general depression symptoms, both summer and winter seasonal depression are characterized by their own set of specific symptoms. 

Symptoms of summer seasonal depression

The typical symptoms of summer seasonal depression are a little different than some other forms of depression. Summer-onset SAD tends to be characterized by irritability instead of low mood. 

These symptoms can include:

Difficulty sleeping (insomnia)

During summer, there are fewer hours of melatonin production, and melatonin is produced later at night. As a result, many people with summer-onset SAD have trouble falling or staying asleep when the days are longer and the nights are shorter.

Loss of appetite

Loss of appetite is a key symptom of summer seasonal depression, which can result in weight loss and lower energy levels. 

Weight loss

Unintentional weight loss is a common symptom of depressive conditions such as summer seasonal depression, likely due to loss of appetite.

Mood and behavior

If you have summer SAD, you may feel anxious and agitated. Some people also find it hard to control their emotions, resulting in them becoming violent toward others. Agitation and irritability may be present instead of low mood.

Symptoms of winter seasonal depression

The symptoms of winter seasonal depression are associated with atypical depression symptoms, including: 

Excessive sleep

Between 70 to 90%⁵ of people with winter seasonal depression sleep excessively and find it difficult to wake up in the morning. 

The shorter days and longer nights during winter mean there are fewer hours of sunlight each day and, therefore, more time for melatonin to be produced, causing excessive sleepiness. 


Unlike summer-onset SAD, which involves loss of appetite, winter-onset involves increased appetite. This can lead to weight gain. 

In particular, 80 to 90%⁵ of people with winter-onset seasonal depression describe having cravings for carbohydrate-rich foods, such as bread, pasta, and baked goods.

Weight gain

About 70 to 80%⁵ of people with winter seasonal depression experience unintentional weight gain. This may result from a combination of eating more due to having an increased appetite and exercising less due to a loss of interest in everyday activities. 

Mood and behavior

A generally low mood and fatigue in the late afternoon are associated with winter seasonal depression. 

Young people at school can also experience a reduction in their cognitive ability and performance. 

Other physical symptoms

Leaden paralysis, a heavy feeling in the limbs, is a physical symptom experienced in people with winter seasonal depression. 

How do melatonin and serotonin affect seasonal depression?

Two chemicals in the brain impact important regulatory processes such as sleep and mood, which are associated with the symptoms of seasonal depression. 


One of these chemicals is melatonin, a hormone produced by the pineal gland in the brain. Its production is triggered at night (darkness) and inhibited during the day (light). 

Melatonin regulates the sleep-wake cycle and induces fatigue, relaxation, and sleep when present in optimal amounts. This helps us to sleep well each night.


Serotonin is a neurotransmitter that sends signals between nerve cells and helps to regulate mood. Serotonin production in the brain is thought to be affected by the intensity and duration of light exposure. This means that the length of day and night can influence how much serotonin is produced. 

Inadequate light exposure can result in receptors in the brain reabsorbing the serotonin that is produced, which leads to low mood. 


As seasonal depression is a mental health disorder, your doctor will assess both your physical and mental symptoms to diagnose you. Since seasonal depression is much like other forms of depression, you need to meet specific criteria to determine whether you have SAD

These criteria include:

Experiencing symptoms

  • You need to display the symptoms of major depressive disorder and/or seasonal depression during a specific season.

  • The onset of these symptoms should not be related to stressors, such as having yearly financial stress during a certain season.

Recurrence of symptoms

  • Seasonal depression is a recurring disorder, but not everyone with seasonal depression experiences a return of their symptoms every single year.

  • However, to be diagnosed with SAD, you need to experience the same seasonal pattern of depressive symptoms for at least two consecutive years. 

Seasonal pattern of symptoms

If you experience depressive symptoms that extend beyond the season they first occurred, you may have a more generalized form of depression. In this case, your doctor may give you a different diagnosis.

Previous depressive episodes

Suppose you have had depressive episodes in the past. In that case, the new seasonal depressive episodes need to be more frequent than any previous non-seasonal depressive episodes to qualify as seasonal depression. 

It can be difficult to assess the severity of your symptoms. A less severe form of seasonal depression is called subsyndromal seasonal depression, more commonly known as seasonal “blues.” It is important to be able to differentiate between these conditions, as it can influence your treatment. 

Using symptoms to screen for seasonal depression

Seasonal pattern assessment questionnaire

A helpful screening tool to determine whether you have seasonal depression is the seasonal pattern assessment questionnaire (SPAQ). You can fill out this form for free online, although it is best to complete it under the guidance and knowledge of a trained medical professional. 

Based on your answers, you will be given a “seasonality score” made up of six behavior ratings between 1 to 24. 

If you score higher than 11, you may have symptoms associated with seasonal depression — but it doesn’t mean that you definitely have the condition. 

If you do have a high score, it is recommended that you speak to your doctor so they can undertake a more comprehensive assessment.

Questions used to calculate the seasonality score on the SPAQ include:

  • On a scale of 1 to 4, with 1 being no change and 4 being an extremely marked change, how much do your sleep length, social activity, overall wellbeing and mood, weight, appetite, and energy levels change with the seasons?

  • What months are the best and worst in terms of how you feel, how much weight you gain, how much you sleep, how much you eat, how much weight you lose, and how much you socialize?

  • How much does your weight fluctuate over the course of a year?

  • How many hours of sleep do you get in each of the four seasons?

  • Do you experience a change in food preference during different seasons?

  • If you experience mood changes with seasons, are they a problem for you?

  •  If so, are these mood changes mild, moderate, or severe?

Patient Health Questionnaire

Another tool used is the Patient Health Questionnaire Nine Item (PHQ-9). This is a screening tool for diagnosing symptoms of general depression and response to treatment, but it's also used for diagnosing seasonal depression. Your doctor will also ask you additional questions about your pattern of depression to diagnose you with seasonal depression.

Each question is based on the DSM-5 criteria. Your answers should be based on your own observation of symptoms, not what other people notice about you. 

Each answer is assigned a score based on how often you experience a particular symptom in question: 

  • Not at all (0 points)

  • Several days (1 point)

  • More than half the days ( 2 points)

  • Nearly every day (3 points 

A PHQ score above 20 can indicate severe symptoms of seasonal depression that should be referred immediately to a mental health specialist for treatment.  

A PHQ score between 10 and 19 indicates moderate to moderately severe symptoms that may require intervention, especially if they significantly impact your daily life. 

Risks if symptoms are not treated

If you do not seek treatment for your SAD symptoms, or if you do not receive the appropriate treatment, complications can arise.

Mental health problems

  • Substance abuse

  • Anxiety

  • Thoughts about suicide 

  • Social withdrawal 

Relapse of seasonal depression

A relapse of seasonal depression symptoms can be more severe than the initial symptoms, which can happen if you stop taking prescribed medications too early. To prevent this from happening, always follow your doctor's advice about taking medications or following a treatment plan. 

If you wish to stop taking medication due to side effects or for any other reason, speak with your doctor so they can help you do this safely and seek alternatives.

Seeking help

If you develop symptoms that could be associated with seasonal depression, it is important to see your doctor to receive a diagnosis and the appropriate treatment plan for your symptoms. This is important regardless of the severity of your symptoms because they can worsen over time if left untreated.

Your family doctor should be your first contact. They may refer you to a specialist such as a psychiatrist or recommend seeing a therapist.

If you have thoughts of suicide or hurting yourself, seek immediate medical attention.

The lowdown

Symptoms of seasonal depression are similar to other depressive disorders. There are two types of seasonal depression: summer-onset and winter-onset.

Each type affects the same functions and processes in the body, such as weight, appetite, and sleep, but in different ways.

As symptoms can range from mild to debilitating, it is important to seek support no matter how minor or severe your symptoms may seem, as any symptoms can impact your quality of life.

Have you considered clinical trials for Seasonal affective disorder (SAD)?

We make it easy for you to participate in a clinical trial for Seasonal affective disorder (SAD), and get access to the latest treatments not yet widely available - and be a part of finding a cure.

Discover which clinical trials you are eligible for

Do you want to know if there are any Seasonal affective disorder (SAD) clinical trials you might be eligible for?
Have you taken medication for Seasonal affective disorder (SAD)?
Have you been diagnosed with Seasonal affective disorder (SAD)?