As daylight hours decrease with the approach of winter, people vulnerable to seasonal depression may experience changes in mood, energy levels and mental well-being.

Millions of people worldwide are estimated to experience seasonal affective disorder (SAD), a form of depression that typically begins in autumn and improves during spring or summer.

Reduced exposure to daylight is considered a key trigger, while a milder form, known as subsyndromal SAD, can also cause seasonal changes in mood. Although less common, seasonal depression can occur during summer as well.

The condition was first described in 1984 by a research team led by Dr Norman Rosenthal, then a researcher at the National Institutes of Health (NIH).

What causes seasonal depression?

Scientists have yet to establish a single cause of seasonal affective disorder. According to the NIH, women are significantly more likely to experience the condition than men.

Symptoms resemble those of other forms of depression and may include persistent hopelessness, pessimism, difficulty concentrating, memory problems and trouble making decisions.

Researchers are investigating how specialised cells in the eyes detect blue wavelengths of light and send signals to parts of the brain that regulate alertness and mood.

Sunlight contains significant amounts of blue light, which helps activate these brain regions and promotes wakefulness.

Researcher Kathryn Roecklein of the University of Pittsburgh found that people with SAD were generally less sensitive to blue light than those without the condition, particularly during winter. The findings suggest that differences in how people respond to light may contribute to seasonal depression.

Light therapy may help manage symptoms

Light therapy is among the commonly recommended treatments for seasonal affective disorder.

Dr Paul Desan of Yale University’s Winter Depression Research Clinic said many people with the condition benefit from using specially designed bright-light devices.

The treatment generally involves sitting in front of a light therapy device for around 30 minutes every morning. These devices produce light at approximately 10,000 lux, significantly brighter than ordinary indoor lighting.

Research suggests that regular morning sessions can help manage symptoms, including milder forms of winter-related low mood.

Such devices typically cost between $50 and more than $400. However, Desan cautioned that some products marketed for seasonal depression may not provide sufficient brightness to be effective.

Yale University has tested several devices and published recommendations, while the nonprofit Center for Environmental Therapeutics offers a consumer guide to help people choose suitable products.

Therapy and medication offer other treatment options

Antidepressants are among the first-line treatments for seasonal affective disorder, alongside light therapy.

Doctors also recommend maintaining a consistent sleep schedule and spending time outdoors, including taking walks on cloudy days.

However, the benefits of light therapy may decline after users stop the treatment.

Researcher Kelly Rohan of the University of Vermont said studies have found that cognitive behavioural therapy (CBT), a form of talk therapy, can provide longer-lasting benefits.

CBT involves working with a therapist to identify and change negative or unhelpful thinking patterns.

The approach can also help people gradually reintroduce enjoyable activities into their daily routines.

Experts suggest planning simple activities to reduce seasonal withdrawal, such as spending time outdoors, meeting friends for coffee and taking part in social activities.