What Is Seasonal Affective Disorder?


Do you often feel down and less energised during the winter months but these feelings disappear when the weather warms up? If yes, you may suffer from seasonal affective disorder (SAD).

Seasonal affective disorder (SAD) or seasonal depression is a type of depression that follows a seasonal pattern. It is often referred to as ‘winter depression’ because the symptoms usually appear during the winter and improve in the summer. However, you can also experience SAD in the summer.1

Around 2 million people in the UK are affected by seasonal affective disorder every year. It often affects people assigned female at birth (PAFAB) rather than people assigned male at birth (PAMAB) and is more prevalent in younger adults than older adults. It is a serious condition that can severely affect your mental health and well-being.1

The symptoms of SAD include:

  • A low mood that is persistent
  • Loss of interest in everyday activities
  • Irritability 
  • Lack of energy and feeling sluggish
  • Feelings of guilt, hopelessness and worthlessness
  • Sleeping too much
  • Gaining weight, overeating and craving carbohydrates
  • Finding it difficult to concentrate
  • Suicidal thoughts

The symptoms specific to summer-onset seasonal affective disorder are:

  • Having trouble sleeping
  • Poor appetite
  • Anxiety or agitation
  • Weight loss
  • Being irritated easily

Whilst some people may have mild symptoms, others may experience severe symptoms that require immediate attention.

Causes of seasonal affective disorder

The exact cause of SAD  is unknown but it is often linked to the decreased exposure to sunlight during the winter months.

Biological factors

The hypothalamus is a part of the brain that is stimulated by light and controls your mood, appetite and sleep. If you have seasonal depression, lack of sunlight prevents the hypothalamus from working properly which affects the:1

  • Production of melatonin
  • Production of serotonin and
  • the body’s internal clock (circadian rhythm)


Melatonin is a hormone that plays a role in sleep patterns and mood. When it is dark, a small gland in your brain called the pineal gland produces melatonin to make you feel sleepy. When it is light, the retina converts that information to the hypothalamus which then sends a message to the pineal gland so it stops producing melatonin.1

Lack of sunlight in the winter means that some people may produce more melatonin than normal which results in seasonal depression symptoms such as low mood, low energy levels and sleepiness.1


Serotonin is another hormone that also affects sleep and mood as well as appetite. Lack of daylight hours leads to lower serotonin levels. If you have seasonal depression, your serotonin levels may be lower than normal which can result in SAD symptoms such as feelings of depression.1

Circadian rhythm

Your body has an internal clock that uses sunlight to time important biological functions such as sleep, mood, energy levels, digestion and appetite. This is known as a circadian rhythm and it occurs over a 24-hour period. Lack of sunlight during the winter can disrupt your body’s internal clock and lead to symptoms of SAD.1


If you have a family history of depression, you may inherit genes that make you more likely to experience symptoms of SAD.

Other disorders

If you have major depression or bipolar disorder, your symptoms of depression may worsen in particular seasons.

Vitamin D

A deficiency in Vitamin D can worsen symptoms of seasonal depression. This is because Vitamin D promotes the production of serotonin which improves your mood. Vitamin D can be consumed with diet but it can also be produced by the body when it is exposed to sunlight. During the colder months, lack of sunlight may mean some people have lower Vitamin D levels which can affect the serotonin activity and therefore worsen SAD symptoms.1

Environmental factors

Weather and temperature

Most people show symptoms of SAD in colder months but some people may experience symptoms in summer. As well as this, SAD is more common in people who live far from the equator which means there is decreased sunlight during the winter and longer days during the summer.1

Lifestyle factors

SAD can also be influenced by lifestyle factors such as:

  • Lack of indoor light
  • Lack of physical activity
  • Lack of friendship
  • An unhealthy diet

Diagnosis of seasonal affective disorder

Screening questions and evaluation

Your GP may carry out a psychological evaluation and ask you certain questions about your:

  • Symptoms
  • Feelings
  • Thoughts
  • Behaviour patterns and if there is a seasonal change
  • Eating and sleeping patterns
  • Family history of mental health conditions (if any)
  • Personal history that may cause symptoms of depression

Differential diagnosis

Diagnosing seasonal affective disorder may be difficult as the symptoms may be similar to other types of depression and mental health disorders such as:1

  • Bipolar disorder
  • Major depressive disorder

Diagnostic criteria

A SAD diagnosis is usually confirmed if:

  • You experience SAD symptoms at similar times each year for at least 2 years
  • You have periods of depression followed by periods without depression

Treatment of Seasonal Affective Disorder

There are various treatments available for seasonal depression, including light therapy, cognitive behavioural therapy (CBT) and antidepressants.

Light therapy

Light therapy, also called phototherapy, uses a light source to mimic natural sunlight and improves your mood. It involves sitting in front of a special light box within the first hour of waking up each day. This type of therapy improves SAD symptoms by reducing the production of melatonin so you are more energised and awake and increasing the production of serotonin so your mood improves.1

Light therapy is effective at relieving SAD symptoms for most people usually within the first week but it may not be a suitable long-term treatment.


Your GP may prescribe you antidepressants if your symptoms are severe. The preferred type of antidepressants for treating seasonal depression are selective serotonin reuptake inhibitors (SSRIs) which increase the level of serotonin in your brain and improve your mood.

Antidepressants are most effective if they are taken at the start of winter before your symptoms begin and continue until early spring.


Talking therapies such as cognitive behavioural therapy, focus on both psychological aspects and social aspects. 

Cognitive behavioural therapy (CBT)

Cognitive behavioural therapy is a type of talking therapy that focuses on changing the way you think and behave in certain situations. For people with SAD, you may focus on replacing negative thoughts related to a particular season with more positive thoughts.

Cognitive behavioural therapy usually occurs over several weeks or months with a trained therapist and is often a more effective long-term treatment compared to light therapy.

Lifestyle changes

You can make a few lifestyle changes and improve your symptoms by:

  • Making your environment brighter - open your curtains/blinds, trim any tree branches that block sunlight
  • Go outside (even on cold or cloudy days) - take daily walks in the sun, eat lunch on a park bench
  • Exercise regularly - physical activity can help relieve SAD symptoms by reducing stress and anxiety
  • Create a regular sleeping schedule - go to sleep and wake up at the same time each day and eliminate napping
  • Take a trip to a warmer country during the winter months

Prevention of seasonal affective disorder

There is no sure way of preventing seasonal affective disorder but you can take steps early on to manage your symptoms and prevent them from getting worse over time.

Education and awareness

It is important to stay educated on seasonal affective disorder so you can identify and manage your symptoms or support someone else who is suffering from this condition. You can do this by taking note of:

  • Your symptoms
  • When your symptoms start and end
  • Whether your symptoms are interfering with your everyday activities
  • What improves or worsens your symptoms
  • Whether you have any other physical or mental health condition

Lifestyle changes

You can make a few lifestyle changes and improve your mental health by:

  • Eating a healthy, balanced diet
  • Exercising regularly
  • Sitting near windows when you are indoors
  • Getting outside in natural sunlight
  • Making your home environment as bright and airy as possible
  • Managing your stress
  • Keeping in contact with friends and family

Supportive interventions

Here are a few useful links for support:


In summary, seasonal affective disorder (SAD) is a type of depression that is triggered by a seasonal change and symptoms usually begin around late autumn and end in early spring. Symptoms include a low mood, oversleeping, loss of interest in everyday activities and weight gain. SAD is often caused by a range of factors, but research shows that it is mainly caused by lack of sunlight. This contributes to a change in our biological clock (circadian rhythm), overproduction of melatonin and reduced production of serotonin. 

Treatment includes light therapy, which mimics natural sunlight, antidepressants and psychotherapy. There is no sure way of preventing seasonal depression, but you can educate yourself on the symptoms and improve your mental health by making a few lifestyle changes such as exercising regularly, eating a healthy diet, going on walks outside and managing your stress.


  1. Munir S, Abbas M. Seasonal depressive disorder. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 May 5]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK568745/ 
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Suad Mussa

Bachelor of Science – BSc, Biology. Queen Mary University of London

Suad Mussa is a biology graduate with a strong passion for medical writing and educating the public about health and wellbeing.

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