Researchers have now found why heart attacks and strokes are more common in the morning.
The levels of a protein in people's blood that slows the breakdown of clots peaks exactly at 6.30 a.m., the researchers from Boston-based Brigham and Women's Hospital and Oregon Health and Science University have said.
"Our research suggests that the circadian system, or the internal body clock, contributes to the increased risk for cardiovascular events in the morning," Frank Scheer, director of the medical chronobiology programme at Brigham and Women's Hospital, was quoted as saying.
The researchers studied the protein levels in the bodies of 12 healthy adult volunteers for two weeks.
The participants were assessed while their daily routines were de-synchronised from their natural body clocks.
They studied changes in the body's level of the protein Plasminogen activator inhibitor-1 (PAI-1) which inhibits the breakdown of blood clots.
The research found a strict rhythm in body's level of PAI-1 with a peak at about 6.30 a.m.
This morning peak in PAI-1 could help explain adverse cardiovascular events in individuals with obesity, diabetes or hearth disease," warned co-author Steven Shea, director of the Oregon Institute of Occupational Health Sciences.
The findings, published in the journal Blood, indicate that the human circadian system causes a morning peak in circulating levels of PAI-1 independent of any behavioural or environmental influences.