The three main categories of depression are currently known as:

• Major depression, or unipolar depression
• Dysthymia, a lasting, low-level depression
• Bipolar disorder, which is sometimes called manic-depressive illness

If you have ever had a major depression, you will recognize its hallmarks. Work, school, relationships, and other aspects of your life may be derailed or put on hold indefinitely. You feel constantly sad or burdened, or you lose interest in all activities, even those you previously enjoyed. This holds true nearly all day, on most days, and lasts at least two weeks.

During this time, you may also experience at least four of the following signs:

• A change in appetite that sometimes leads to weight loss or gain
• Insomnia or, less often, oversleeping
• A slowdown in talking and performing other tasks or, conversely, restlessness and an inability to sit still
• Loss of energy or feeling tired much of the time
• Feelings of worthlessness or excessive, inappropriate guilt
• Problems concentrating or making decisions
• Thoughts of death or suicide, or suicide plans or attempts

Other signs can include a loss of sexual desire, pessimistic or hopeless feelings, and physical symptoms such as headaches, unexplained aches and pains, or digestive problems.

If you talk to any two people about their experience of depression, you might well think they were describing entirely different illnesses. For example, one might not be able to summon the energy to leave the house, while the other might feel agitated and restless. One might feel deeply sad and break into tears easily. The other might snap irritably at the least provocation. One would pick at food, while the other might munch constantly. On a subtler level, two people might both report feeling sad, but the quality of their mood could differ substantially in depth and darkness.