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."
The short answer: The "high" moods never reach fully manic episodes. (The low, depressive phases can be very intense.) 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.
There is an additional variation 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.
A Huffington Post reader sent along this tip:
This link gives an educational and personal overview of living with bipolar disorder. It was developed with the hope of reducing the stigma of this mental illness through education. http://livingbipolardisorder.blogspot.com/