According to the American Psychiatric Association, Americans who suffer from SAD — which is also known as Major Depressive Disorder with a seasonal pattern in the DSM-5 — tend to have the most difficult time during January and February. It’s believed that SAD is caused by shorter daylight hours and less sunlight — and because of this shift in sunlight, people’s biological internal clock or circadian rhythm is negatively affected, which has the possibility to disrupt their sleep cycle and cause depression symptoms to occur.