Researchers delved into this and discovered that it's all due to your body clock.
The body produces a protein that slows down the breakdown of clots, and a team from Brigham and Women's Hospital, in Boston, and Oregon Health and Science University, discovered that this enters peak production around 6.30am in some people.
"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.
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They studied levels of the protein Plasminogen activator inhibitor-1 (PAI-1) of 12 healthy adult volunteers for two weeks. These people were then asked to change their routine so that their body clocks were desynchronised.
This was to measure whether their protein levels changed accordingly.
he 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, showed that the human circadian system causes a morning peak in circulating levels of PAI-1 independent of any behavioural or environmental influences.
Dr Shea added: ""Indeed, the circadian system determined to a large extent the PAI-1 rhythm observed during a regular sleep/wake cycle. This morning peak in PAI-1 could help explain adverse cardiovascular events in vulnerable individuals."