The symptoms of depression can vary from person to person, but some common signs include persistent feelings of sadness, emptiness, or hopelessness; loss of interest or pleasure in activities once enjoyed; changes in appetite or weight; sleep disturbances; fatigue or low energy; feelings of worthlessness or excessive guilt; difficulty concentrating; and recurrent thoughts of death or suicide. According to the National Institute of Mental Health, these symptoms must persist for at least two weeks to be considered clinical depression. It's important to seek professional help if you or a loved one is experiencing these symptoms.