STOCKHOLM (AP) - Jeffrey Hall, 72, wryly noted that he was already awake when he received the call from Sweden about his Nobel Prize in medicine because of changes in his circadian rhythm as he has grown older.

Speaking from his home in rural Cambridge, Maine, he says "I said 'Is this a prank?' I didn't really believe it. I didn't expect it."

Hall won the Nobel on Monday with fellow Americans Michael Rosbash and Michael W. Young for their discoveries about the body's daily rhythms.

Hall said scientists have known about circadian rhythms since the 1700s. He said understanding the mechanics of the circadian rhythm can provide researchers with an opportunity to address circadian rhythm disorders that contribute to sleep problems.

He says "If you understand how the normal process works, that gives you a chance, not an inevitability, but a chance to influence the internal workings of the clock and possibly to improve a patient's well-being."