SAD is believed to be a mood change caused by melatonin levels changing in the body due to the way the light changes this time of year. As the days shorten (meaning shorter durations of light) our bodies change as well. Higher melatonin levels tend to help our bodies prepare for sleep, making us tired and ready to slow down, and it increases (or builds up) in the body when there is less light. Melatonin levels decrease when there is more light which tends to suppress the production of melatonin as light enters the eyes. Therefore the higher levels of melatonin in the body during the Fall and Winter months tends to affect some people more severely than others, leading to symptoms of depression. This can include feelings of hopelessness, worthlessness, sluggishness, fatigue, decreased interest in activities, sleep disturbances, poor concentration and depressed feelings for most of each day. It's believed that lower serotonin level as well as lower vitamin D levels also play a role in SAD but the research is still out.
The best plan for SAD is to try preparing from many angles for the time of year you know you will be affected by the seasonal change the most. Taking extra vitamin D, consulting with your doctor about getting on antidepressant medications, setting up regular psychotherapy sessions this time of year with your counselor, and most of all looking into ways to get some "light therapy" which can help suppress the production of too much melatonin.
Both vitamin D and light therapy can be obtained by making sure you don't hunker down inside too much. Getting outside to absorb as much sunlight as you can is important, but it's a little harder since the cold temperatures tend to make us all want to stay indoors as the seasons change. Consider some outdoor activates that might be popular in cooler weather such as skiing, snow shoeing, sledding, and even Fall and Winter hiking. Even just getting out for walks in the daylight, regardless of the temperature, is a good idea. The exercise involved in many of these activities can also help boost your serotonin levels, so it's all-around good therapy.
You can also try many of the various forms of light therapy devices designed to provide your body with light exposure. These "light boxes" are much brighter than regular indoor lights and you can sit in front of them for 30 min to an hour each morning during these darker times of the year. Make sure you get appropriate lights that filter out damaging UV light and are marked as safe for your body and eyes.
You don't have to fall passive to Seasonal Affective Disorder (SAD). Be proactive and do all you can now to prepare for the changing season and changing moods.
"It always takes me by surprise how dark it gets this time of year", ~To a Poet, First Aid Kit
Thanks to Bonnie Moreland for the great photo, https://creativecommons.org/publicdomain/zero/1.0/