Serial entrepreneur Paul English attributes much of his success in building Kayak to an unexpected source: his struggle with bipolar disorder.

In a recent Inc. Founders Forum interview, English says the condition boosted his confidence in his ideas for the travel site, which in turn convinced him to actively pursue them.

"I definitely credit the hypomania with helping me work for long periods of time," he tells Tracy Kidder, author of A Truck Full of Money, which chronicles his accomplishments while dealing with mental illness. Hypomania is an elevated mood state that's similar to the manic state experienced by people with bipolar disorder, except that it's milder and doesn't cause impairment, according to Harvard Medical School.

English relates that he kept odd hours when building the company, due to his mental illness.

"A lot of Kayak was invented at 2 in the morning or 4 in the morning while I was up thinking about how to make things faster and better," he says.

Although the condition ultimately had a positive impact on Kayak, English endured some negative--and downright odd--consequences.

"I almost bought this ridiculous lighthouse in Boston harbor," he tells Kidder, recalling how he thought it would be a great party house despite not having electricity or plumbing.

Believing that English was becoming too manic, a psychiatrist suggested changing his medication--a move that the entrepreneur says worried him.

"If you take someone who is bipolar and you put them on meds to cure the highs it will cut down the creativity," he says. Fortunately English did elect to take the advice of his doctor, who rightly believed the change would lead to a more effective and healthy way of working.

For more of Paul English's story, watch the video above.