Catherine Zeta Jones has checked herself into a mental health facility for treatment of bipolar II disorder, her rep confirmed to CNN on Wednesday. The 41-year-old actress has been by husband Michael Douglas' side since his treatment for throat cancer last fall.

(Health.com) -- Although the symptoms of bipolar disorder can vary significantly from person to person, mental health professionals have identified four main subtypes of the illness that together are referred to as bipolar spectrum disorders: bipolar I, bipolar II, bipolar not otherwise specified, and cyclothymia.

Factors that differentiate the types of bipolar include the duration and intensity of the mood swings. Knowing which type you have can help doctors choose the right course of treatment, according to Gabrielle Carlson, M.D., professor of psychiatry and pediatrics at Stony Brook University Medical Center, in New York.

Bipolar I

People who have bipolar I -- the "classic" bipolar disorder -- have experienced one or more manic episodes lasting at least a week and almost always one or more major depressive episodes.

Manic episodes bring an abnormally elevated mood. A person may be agitated, have grandiose ideas, need less sleep, be easily distracted, and act impulsively.

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Depressive episodes bring feelings of sadness, hopelessness, guilt, worthlessness, and pessimism; patients may experience difficulty concentrating, a loss of interest in normal daily activities, and changes in eating and sleeping habits. It's considered a depressive episode if the person experiences several of these symptoms for most of the day for more than two weeks.

Bipolar disorder can also cause psychosis, which may include hallucinations (seeing things that aren't there) or delusions (strongly held beliefs not based in reality and not influenced by rational thinking).

Men and women are equally likely to have bipolar disorder, although a 2005 study in the "American Journal of Psychiatry" found that men are more likely to have their first manic episode at a younger age. The disease is also evenly distributed among ethnicities, says S. Nassir Ghaemi, M.D., the director of the Mood Disorders Program at Tufts Medical Center, in Boston.

In bipolar disorder, periods of depression typically last longer than manic episodes. Depression can last for a year or longer, while manic episodes rarely go on for longer than a few months. If treatment is successful, bipolar patients may experience months or years of mood stability between episodes, although one-third have some residual symptoms, according to the National Institute of Mental Health (NIMH).

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Bipolar II

Depression is the primary characteristic of bipolar II. While those with bipolar II do have "up" periods, these episodes are less marked. Instead of full-blown mania, people with bipolar II experience hypomania, a milder form of mania. Studies show that women are slightly more likely to have bipolar II.

Though a person with bipolar II may deny that anything is wrong, loved ones will probably notice that he or she seems agitated, is flying off the handle more often, or seems unusually upbeat.

Bipolar II is sometimes mistaken for depression because the hypomanic periods are harder to detect. Over time, without treatment, hypomania -- the "up" period -- can progress into mania or turn into a depressed state.

Bipolar disorder not otherwise specified (NOS)

This is a catchall category for those who seem to have bipolar disorder, but who don't fit neatly into any category.

For an illness to be considered bipolar I, for example, a manic episode needs to last at least a week. If the manic episode lasts only three days, doctors may say you have bipolar disorder not otherwise specified, Carlson says.

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Other bipolar variations

Bipolar disorder is a complex condition that isn't easy to categorize. Some people have bipolar I without ever having experienced a major depressive episode, though this is unusual.

People with bipolar disorder may also experience a mixed episode, symptoms of depression and mania simultaneously.

"If you're manic, you may not be going 100 mph every second of every day," Carlson says. "You may be moody and have ups and downs. You are wired but your emotions are completely dysregulated. Someone tells you they hate your lipstick, and you may burst into tears or hit them in the nose."

And even if you've been diagnosed with a particular type of bipolar, it doesn't mean that your symptoms will remain the same over time, or even that you will remain in the same subtype.

Left untreated, bipolar disorder tends to worsen over time, according to the NIMH. Episodes can be more severe or can begin to cycle rapidly. About 20 percent to 25 percent of people have four or more distinct episodes of mania or depression in a year, according to Ghaemi. This is called rapid cycling, and it can occur in those with bipolar I, II, or NOS. Rapid cycling tends to happen later in the course of the illness and is more common in women than men.

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Even within rapid-cycling bipolar disorder, there are many variables.

While some who are rapid cycling have periods of normality between episodes, a smaller number careen from high to low without any breaks in between; this is sometimes called continuous cycling. An even smaller group has ultra rapid, ultra-ultra rapid, or ultradian cycling, which can bring multiple mood shifts in a single day.

Rapid-cycling bipolar disease poses challenges for physicians trying to determine the correct treatment, because antidepressants can cause manic episodes to flare or get worse.

Be sure to write down the details of your manic episodes, including your symptoms, feelings, and how long the episode lasts, so your doctor is better able to help.

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Cyclothymia

People who have cyclothymia are often considered by their loved ones to be extremely moody. They have a history of cycling through "up" and "down" periods, none of which are so severe or last long enough to qualify as mania or major depression.

People with cyclothymia may have bursts of energy and need less sleep, followed by mild depression. "Very few people have to come to the doctor for treatment for cyclothymia," says Carlson.

Some mental health professionals consider cyclothymia to be a condition distinct from bipolar. But not all mental health professionals agree. Ghaemi says cyclothymia is a personality trait, albeit one that's related to bipolar.

Research shows that people who have a parent or close family member with cyclothymia are more likely to have bipolar disorder. In addition, people with bipolar disorder have a greater tendency to experience cyclothymia between episodes of depression or mania.

"In my view, it's a temperament," Ghaemi adds

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