Depression (major depressive disorder) is a common and serious medical illness that has a negative impact on how you feel, think, and act. It is, thankfully, treatable. Depression causes feelings of sadness and/or a loss of interest in previously enjoyed activities. It can cause a variety of emotional and physical problems, as well as a reduction in your ability to function at work and at home.
Sadness or a depressed mood
Loss of interest or pleasure in previously enjoyed activities
Appetite changes — weight loss or gain unrelated to dieting
Sleeping difficulties or excessive sleep
Energy loss or increased fatigue
Increase in meaningless physical activity (e.g., inability to exercise)
Increase of intentionless physical activity (e.g., inability to sit still, pacing, handwringing) or slowed movements or speech (these actions must be severe enough to be observable by others)
Feeling insignificant or guilty
Problems thinking, concentrating, or making decisions
Suicide or death thoughts
For a depression diagnosis, symptoms must last at least two weeks and represent a change in your previous level of functioning.