The clocks went back on November 1, and for much of the country the sun now sets before 5 p.m. Many people notice they feel more tired, less motivated or simply flat at this time of year. For most, it is a mild dip often called the winter blues. For some, it is seasonal affective disorder, or SAD, a form of depression that returns at the same time each year.

Knowing the difference matters, because SAD is common and treatable.

What seasonal affective disorder is

The National Institute of Mental Health describes SAD as a type of depression with a seasonal pattern. In the most common form, symptoms start in late fall or early winter and ease in spring and summer. A less common form follows the opposite pattern, with symptoms in summer.

SAD is not just disliking winter. To be diagnosed, a person must have symptoms of major depression that recur during specific seasons for at least two years.

Signs to watch for

Winter-pattern SAD includes the symptoms of depression, such as:

  • Feeling sad, empty or hopeless most of the day, nearly every day.
  • Losing interest in activities you usually enjoy.
  • Low energy and difficulty concentrating.
  • Feeling worthless or guilty.

It also often includes symptoms that are more typical of the winter pattern:

  • Sleeping much more than usual.
  • Craving carbohydrates and eating more, often with weight gain.
  • Pulling away from friends and social life, sometimes described as feeling like hibernating.

Who is more likely to have it

SAD is more common in women than men, more common in people who live farther from the equator, where winter days are shorter, and more common in people who already have depression or bipolar disorder, or who have relatives with these conditions. It often begins in young adulthood.

Why shorter days affect mood

Researchers believe reduced daylight disrupts the body’s internal clock and affects chemicals that regulate mood and sleep, including serotonin and melatonin. Less sunlight may also mean lower vitamin D levels, although whether that contributes to SAD is still being studied.

Treatments with evidence

SAD responds well to treatment, and several options are often combined:

  • Light therapy. Sitting near a special bright light box, typically for about 30 minutes each morning, can relieve symptoms for many people. These boxes are much brighter than ordinary lamps and filter out ultraviolet light. People with eye conditions or bipolar disorder should talk to a doctor before starting.
  • Talk therapy. A form of cognitive behavioral therapy adapted for SAD helps people change thought patterns and schedule activities that counter the urge to withdraw. Studies suggest its benefits can last into future winters.
  • Medication. Antidepressants can help, and some people with a clear seasonal pattern start them before symptoms usually begin.
  • Vitamin D. It is sometimes recommended, especially for people who are low in it, but the evidence that it treats SAD on its own is mixed.

Everyday habits that help

These won’t replace treatment for real depression, but they help many people through darker months:

  • Get outside in daylight, especially in the morning, even when it’s cloudy.
  • Keep a regular sleep and wake time, including on weekends.
  • Stay physically active; even short walks help.
  • Plan social contact on purpose, because the pull to stay home is part of the problem.
  • Open curtains and sit near windows during the day.

When to get help

Talk to a doctor or mental health professional if low mood lasts more than two weeks, if it affects your work, relationships or daily life, or if it returns every winter. If you or someone you know is thinking about suicide, call or text 988 to reach the 988 Suicide & Crisis Lifeline, available 24 hours a day.

What we think

Seasonal depression is one of the few mental health conditions that is predictable: if it happened last winter, it may happen again. That makes it unusually possible to prepare.

If the last few winters were hard, don’t wait for the darkest weeks of January. Talk to a doctor now about light therapy or other treatment, and build daylight and routine into your days before the slump arrives.

This article is general information, not medical advice. If you have symptoms or questions about your own health, talk to a doctor, nurse or pharmacist. In an emergency, call 911.