New research suggests that more than 40 per cent of Americans suffering from a major depression could have recurring episodes of manic behavior.

These patients reportedly suffer from sub-threshold hypomania or a milder form of mania that usually lasts for up to four days and is thus not considered to be a bipolar disorder, the researchers who conducted the study suggest.

A team at the U.S. National Institute of Mental Health (NIMH) worked with data from a survey involving more than 5,000 American households to find that people with this mild form of mania had higher rates of anxiety and substance abuse besides going through more depressive episodes.

Lead author Kathleen Merikangas, a senior investigator at the NIMH in Bethesda, said with this disease the patients usually become more active and energetic, tend to sleep less and get agitated more easily and more frequently.

The study indicates that patients of sub-threshold hypomania were considerably more anxious than depressed people. It was also suggested that these patients had a family history of mania as was the case with people with bipolar disorder.

Though these manic disorders lasted only two or three days, they recurred frequently enough but pointed out that some periods of high energy were actually normal. The results of the study have been published in the latest issue of the American Journal of Psychiatry.

"If someone has one instance where she feels really good for a few days because of something that happened in her life, like falling in love, that's not sub threshold hypomania," Merikangas says in the online edition of the medical journal.