While the exact cause of SAD is not fully known, the condition is often linked to lack of sunlight during the shorter autumn and winter days. It is thought that shorter days and less daylight may trigger a chemical change in the brain leading to symptoms of depression.
Biological clock change:
Your biological clock regulates your mood, sleep and hormones. When there’s less sunlight, you’re out of step with the daily schedule you’ve been used to and you find it difficult to adjust to changes in daylight length.
Exposure to sunlight is thought to release the production of serotonin in your brain, which contributes to feelings of happiness. Decreased sun exposure has been associated with a drop in your serotonin levels in the winter which can lead to seasonal affective disorder depression with seasonal pattern.
Vitamin D deficiency:
Because sunlight helps produce vitamin D, less sun in the winter can lead to a deficiency, which can affect your serotonin level and your mood.
Melatonin effects your sleep patterns. The lack of sunlight in autumn and winter can stimulate an overproduction of melatonin, making you feel sluggish and sleepy.