Someone diagnosed with manic depression may swing from moods of deep depression to periods of overactive, excited behaviour known as mania. Between these severe highs and lows there may be relatively stable times; although this isn't always the case. Some people see or hear things that others around them don't (known as visual or auditory hallucinations or delusions).
Everybody has their ups and downs in daily life, but in manic depression these changes are extreme. During the manic phase, people may feel euphoric, full of a sense of their own importance and brimming with ambitious schemes and ideas. They may spend money extravagantly, and build up debts. They may eat and sleep very little, and talk so quickly that it's difficult to understand them. They may be easily irritated and suffer from periods of anger.
A person may be quite unaware of these changes in their attitude or behaviour. After a manic phase is over, they may be quite shocked at what they've done and the effect that it has had. On the positive side, people can be very creative during mania, and may feel that it's a very valuable experience.
Mania may flare up periodically, but depression is the most consistent symptom. People may feel overwhelming despair, guilt and worthlessness. Problems concentrating and remembering things can make life very difficult and undermine the simplest tasks. The experience of manic depression may also provoke suicidal feelings.