Strokes more likely to occur early morning and early evening

Japanese scientists have found that strokes are most likely to occur during certain two-hour periods; one in the morning and the other in the evening.

The researchers, from Iwate Medical University, looked at patients suffering from three different types of stroke for the first time and the most common was an ischaemic stroke, which is caused by restricted blood flow to the arteries in the brain; strokes cause brain tissue to die.

They examined 12,957 cases, looking for patterns in the wake-sleep cycle and found the risk peaked between 6am and 8am and 6pm and 8pm and the risk was lowest during sleep.

The researchers divided a full day into two 12-hour periods and say the internal body clock appears to affect the timing, with the lowest risk being when patients are asleep and there are changes to the blood and circulation.

Haemorrhagic strokes are less common and they happen when a blood vessel bursts inside the brain. There are two types of Haemorrhagic strokes, one where bleeding is directly into the brain tissue (intracerebral), and one where bleeding is in the arteries on the brain's surface (subarachnoid).

The researchers found that the risk peaked for all three types of stroke during the morning and early evening.

It appears ischaemic strokes are more likely to occur in the morning, and slightly less likely to occur during the evening peak slot, while haemorrhagic strokes showed less of a peak in the morning, but a higher peak in the evening.

The researchers suggest that fluctuation in blood pressure is likely to be a significant cause of the peaks and troughs in risk and other research backs this theory.

They also believe other properties of the blood may increase the risk of an ischaemic stroke, and decrease the risk of a haemorrhagic stroke in the morning; the platelets, the tiny solid particles found in blood are known to stick together more easily and possibly form a clot, in the morning and the blood also tends to be thicker at this time.

When blood is less thick and sticky, excessive bleeding is more likely, raising the risk of a haemorrhagic stroke.

The study also found that a fifth of ischaemic strokes were much more likely to occur during sleep than haemorrhagic strokes and most were concentrated in the period immediately before waking up, although the stroke probably started earlier.

The study appears in the Journal of Neurology, Neurosurgery and Psychiatry.