It is more dangerous to have a heart attack in the morning than at any other time of day, according to research.
Patients who had an attack between 6am and noon suffered around 20% more damage to their hearts compared with other times.
Researchers believe the pattern is connected to circadian rhythms, the 24-hour “body clock” processes that influence many biological functions.
Before this study, heart attacks were known to occur more frequently after a person wakes up, but it was unclear how much time affected their severity.
A group of Spanish researchers analysed data on 811 patients admitted to the Hospital Clinico San Carlos in Madrid to determine the impact of the time of day when a heart attack happens.
They examined the amount of dead heart tissue left by attacks, with the size of the dead area, or “infarct”, calculated by looking the enzyme levels in patients.
Timing of heart attacks was divided into four six-hour periods running in phase with 24-hour circadian rhythms.
Patients with the largest infarct size were those whose heart attacks occurred in the dark-to-light transition period between 6am and noon.
In total, 269 patients had their heart attack during the morning danger period. Another 240 suffered attacks between noon and 6pm, 161 between 6pm and midnight, and 141 between midnight and 6am.
The findings have been published online in the journal Heart.
The team wrote: “Significant circadian oscillations in infarct size were found in patients according to time of day of Stemi onset.
“The infarct size was found to be significantly larger with Stemi onset in the dark-to-light transition period (6am-noon).
“If confirmed, these results may have a significant impact on the interpretation of clinical trials of cardioprotective strategies in Stemi.”