Catherine Zeta Jones' Bipolar Disorder: What Is Bipolar II?

Catherine Zeta Jones shocked many this week when it was announced the actress was diagnosed withbipolar disorder and sought treatment at a mental health clinic.

Catherine Zeta Jones' publicist said that "after dealing with the stress of the past year," she "made the decision to check into a mental health facility for a brief stay to treat her bipolar II disorder."

But what is bipolar II, and how is it different thanbipolar I disorder?

The short answer: it's typically less severe. People with bipolar II disorder experience high and low mood swings, but the highs are never fully manic episodes.

From: WebMD:

In bipolar II disorder, the "up" moods never reach full-on mania. The less-intense elevated moods in bipolar II disorder are called hypomanic episodes, or hypomania. A person affected by bipolar II disorder has had at least one hypomanic episode in life.

Additionally, there is an even milder form of the disorder, called Cyclothymic Disorder or Cyclothymia. It's defined by the National Institute of Mental Health as "episodes of hypomania that shift back and forth with mild depression for at least two years."

A recent report showed about 2.4 percent of people around the world have had a diagnosis of bipolar disorder at some point in their lifetime; the U.S. has the highest lifetime rate, at 4.4 percent.

Scientists are still researching what causes bipolar disorder, but are looking into several risk factors, including genetics and brain structure. Symptoms usually start developing during the teens or twenties, and nearly always before age 50, reports WebMD.