What is the difference between bipolar I and bipolar II disorder?

Bipolar I disorder is defined by at least one manic episode, a period of abnormally elevated or irritable mood with increased energy lasting at least seven days, accompanied by symptoms like decreased need for sleep, rapid speech, racing thoughts, grandiosity, and risky behavior. Depressive episodes also commonly occur. Bipolar II disorder is defined by at least one hypomanic episode, a less severe form of mania lasting at least four days, and at least one major depressive episode.