In addition to past research that shows that heart attacks are more common in the morning hours, new research shows they’re also more dangerous at this time of day. In fact, scientists discovered that if a heart attack happens in the morning, the patient suffers about 20% more damage to their cardiovascular muscle.

Scientists still don’t know exactly why more damage is done in the morning, but they think it may be due to the fact that in the morning, hormone levels, blood pressure and metabolism rates go up significantly as compared to the night hours.

For example, blood pressure is at its lowest when sleeping, but reaches its highest point in the morning. Metabolism is low at night but increases when a person wakes from sleep, and cortisol, a hormone that increases levels of blood sugar, is in the body at higher concentrations in the morning than it is at night.

The National Centre of Cardiovascular Research based in Madrid, Spain looked at 811 patients between 2003 and 2009 who were checked into the hospital for having a heart attack. The results, which were published in the journal Heart, showed that 269 had a heart attack between 6 a. m. an noon, 240 between noon and 6 p. m., 161 from 6 p. m. to midnight, and an additional 161 from midnight to 6 a. m.