As winter approaches and daylight hours decrease, people vulnerable to seasonal depression may begin experiencing changes in their mood, energy levels and mental well-being.

Millions of people worldwide are estimated to experience seasonal affective disorder (SAD), a form of depression that generally begins in autumn and improves during spring or summer. The condition can be triggered by reduced daylight, including the shift back to standard time, which occurred Sunday across much of the United States, resulting in darker afternoons.

A milder form, known as subsyndromal SAD, is also recognised by medical professionals. Seasonal depression can also occur during summer, although researchers know less about this form.

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), who introduced the term seasonal affective disorder.

"I believe that because it is easy to remember, the acronym has stuck," Rosenthal said.

Scientists investigate the causes of seasonal depression

Researchers have yet to identify a single cause of seasonal affective disorder. According to the NIH, women are affected significantly more often than men.

Its symptoms resemble those of other forms of depression and may include persistent hopelessness or pessimism, along with difficulties concentrating, remembering information and making decisions.

Scientists are investigating how specialised cells in the eyes detect blue wavelengths of light and convert them into neural signals that influence mood and alertness.

Sunlight contains substantial amounts of blue light. When these specialised cells absorb it, they activate areas of the brain responsible for alertness, helping people feel more awake and potentially improving their mood.

Kathryn Roecklein, a researcher at the University of Pittsburgh, studied how the eyes of people with and without SAD respond to blue light.

Her research found that people with SAD were generally less sensitive to blue light than those without the condition, particularly during winter. The findings point to a possible explanation for winter-related depression.

Bright light therapy may help during shorter days

Many people experiencing seasonal affective disorder benefit from light therapy, said Dr Paul Desan of Yale University's Winter Depression Research Clinic.

The treatment involves sitting in front of a specially designed bright-light device for approximately 30 minutes each morning. These devices produce light around 20 times brighter than ordinary indoor lighting.

Research supports using light therapy devices with a brightness of approximately 10,000 lux, a standard measurement of light intensity. Studies indicate that using them for 30 minutes every morning can help manage symptoms.

According to Desan, the approach may also benefit people experiencing milder forms of winter-related low mood.

Specialised light therapy devices typically cost between $50 and more than $400. However, Desan warned that some products marketed for seasonal affective disorder may not produce sufficiently bright light to be effective.

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

People diagnosed with SAD should also contact their insurance providers to determine whether the cost of a light therapy device is covered.

Talk therapy and medication offer additional treatment options

Antidepressant medications 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 diminish once people stop using the devices.

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

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

According to Rohan, therapy can also help individuals gradually reintroduce enjoyable activities into their daily routines.

Planning simple, pleasurable activities can help people break out of seasonal withdrawal. These may include going outside, meeting a friend for coffee or participating in other enjoyable social activities.