Periods of severe depression interspersed with periods of mania or hypomani (depending on if you have type I or type II). Mania is essentially the opposite of depression. Excess energy, racing thoughts, strange ideas, feeling powerful and self important and invulnerable, needing less sleep, overspending or excessive gambling are often signs of mania/hypomania. (hypomania is less severe than mania but includes some of the symptoms)