EVERYONE has had times when they felt sad or down for a while. Usually these feelings pass pretty quickly and life goes back to normal. However, for some people, these sad feelings linger, and this may be a sign of depression.

Although men and women can suffer from depression, it's more common in women. The reasons for this are uncertain. Some kinds of depression--such as postpartum depression--are linked to women's hormones. But women are more susceptible to all forms of depression.

Depression is a persistent feeling of sadness or low mood accompanied by other symptoms such as diminished or increased appetite, difficulty sleeping or sleeping too much, fatigue and irritability. There are different forms of depression, including:

Major depression

Dysthymia

Seasonal affective disorder

Premenstrual dysphoric disorder

Postpartum depression.

Major depression is a disabling form of depression that can interfere with a person's ability to function normally. Everyday activities like working, studying and just generally enjoying life become impossible. Dysthymia is a chronic form of depression that is not as severe as major depression. It is not disabling, but it can be very difficult to function normally, and it is long-lasting (two years or more).

People with dysthymia are more likely to experience an episode of major depression. When this occurs, it's called double depression. Some people experience depression only during certain times of the year. This is the case with seasonal affective disorder, which is characterized by the onset of depression symptoms during the winter months when there is less natural sunlight.

And then there's premenstrual dysphoric disorder, which also occurs only at certain times. HORMONAL CHANGES - Many women experience premenstrual syndrome with bloating, mood swings and irritability that can result from the hormone changes occurring just before the period.

Premenstrual dysphoric disorder is a more severe form of PMS, and it affects a woman's ability to function. The symptoms of PMDD occur during the week or so before the period, and go away once the period starts.

The fluctuations in hormones and increased responsibility that occur after childbirth also can lead to depression in some women. Called postpartum depression, it is more severe than the brief episode of mild mood changes that some women feel that is often called "baby blues."

Postpartum depression is much more serious and longer-lasting, and requires treatment and support. As researchers learn more about depression, it's becoming apparent that the disease affects more than just mood or behavior. Depression has been linked to a higher risk of heart disease, and many people with chronic or serious diseases will suffer from depression.

GETTING HELP - The good news is that depression in all its forms can be treated. Most forms of depression will respond to some combination of medication and psychotherapy, or in the case of SAD, light therapy. The combination of medication and therapy usually works better than either alone, but some conditions, such as PMDD, may respond well to just medication.

Depression, like other mental illnesses, often goes undiagnosed and can be a source of stigma and embarrassment for those affected. It need not be. May is Mental Health Month, and there's no better time to become more aware of this very common problem and to get help if you or a loved one are suffering from depression.