While the holidays are merry for many, for some they can be hard to get through. Depression and seasonal affective disorder, or SAD, are common this time of year. Psychiatrists estimate together they affect about 13 million Americans.

According to the American Psychiatric Association, depression is a common and serious medical illness that negatively affects how you feel, the way you think and how you act.

Depression affects an estimated one in 15 adults in a given year.

While depression affects people all the time, SAD only affects people with feelings of depression during certain times of the year. In addition to stress, weather changes, loneliness, fatigue and the loss of a loved one can trigger SAD during the holiday season.

"If it's like a mom or a dad who was instrumental in how that holiday went, losing that person and losing the holiday the way it used to be is a very, very sad thing," Seton Mind Institute's Kim Kjome said.

The symptoms for both depression and SAD are similar. If you notice negative thoughts creeping in, low energy or your sleeping and eating habits changing, you might be affected. Kjome said that's when it's a good time to see a doctor. Treatment can range from regular therapy to medication or lifestyle changes, such as seeking out more support.

Since many people develop SAD in the winter when there's not as much sunlight, light therapy can be helpful.

"It produces serotonin and melatonin at different times during the day which are appropriate," Kjome said. "If we don't have enough sunshine sometimes people's brain's don't make enough of those chemicals."

Psychiatrists said SAD is more common in women than in men. It's rare in people under the age of 20.