Getting out of bed before 5 a.m. might be bad for your health, according to a study in Japan that found early risers have a higher risk of medical conditions that can lead to heart attack and stroke.

People who habitually rose before 5 a.m. had a 1.7 times greater risk of high blood pressure and were twice as likely to develop hardening of the arteries as those who got up two to three hours later, researchers in Kyoto found. The study, of 3,017 healthy adults aged 23 to 90, also found a possible link between vascular disease and early birds who began the day with vigorous exercise.

"The results are contrary to the commonly held belief that early birds are in better health," said Mayuko Kadono, a physician at Kyoto Prefectural University of Medicine, who led the study. "We need to find what the causes of this are, and whether exercising after waking early is beneficial."

Cardiovascular disease, which includes heart attack, stroke and hypertension, is the biggest cause of death globally, according to the World Health Organization. The WHO estimates 20 million people may die from cardiovascular disease every year by 2015, compared with about 17.5 million a decade earlier.

Kadono's results were presented at the Fifth Congress of the World Federation of Sleep Research and Sleep Medicine Societies, which is being held in Cairns, Australia, from Sept. 2 to 6.