Morning 'worst heart attack time'
Heart attacks are more dangerous in the morning than at any other time of day, research has shown.
Compared with other times, patients who had an attack between 6am and noon suffered around 20% more damage to their hearts.
The pattern is thought to be linked to circadian rhythms, the 24-hour "body clock" processes that influence many biological functions.
Heart attacks were known to occur more often when a person awakes from sleep, but it was unclear to what extent timing affected their severity.
A team of Spanish researchers set out to determine the impact of the time of day when a heart attack happens.
Analysing data on 811 patients admitted to the Hospital Clinico San Carlos in Madrid, they measured the amount of dead heart tissue left by attacks. The size of the dead area, or "infarct", was calculated by looking at levels of enzymes in the 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 were published online in the journal Heart.