LONDON (BBC Health News) -- Strokes are most likely to occur during two two-hour periods, one in the morning, and the other in the evening, research suggests.

Japanese scientists, who examined 12,957 cases, found the risk peaked between 6am and 8am and 6pm and 8pm. Risk was lowest during sleep.

They said the key was likely to be changes to the blood and circulation governed by internal body clock.

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

The researchers, from Iwate Medical University, looked a patients suffering three different types of stroke.

Most common was an ischemic stroke, caused by restricted blood flow to the arteries in the brain.

Hemorrhagic strokes, which are less common, occur when a blood vessel bursts inside the brain. There are two types: bleeding directly into the brain tissue (intracerebral), and bleeding in the arteries on the brain's surface (subarachnoid).

In all three cases, the researchers found that the risk peaked during the morning and early evening.

Ischemic strokes were particularly likely to occur in the morning, and slightly less likely to occur during the evening peak slot.

Hemorrhagic strokes showed less of a peak in the morning, but a higher peak in the evening. -------------------Sleep

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

Previous studies have shown that blood pressure tends to be highest during the morning, and often peaks again during the evening.

However, they believe other properties of the blood may increase the risk of an ischemic stroke, and decrease the risk of a hemorrhagic stroke in the morning.

For instance, platelets, the tiny solid particles found in blood are known to stick together more easily -- and possibly form a clot -- in the morning. 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 hemorrhagic stroke.

The study also found that one type of stroke -- ischemic strokes -- were much more likely to occur during sleep than hemorrhagic strokes.

Around a fifth of ischemic strokes occurred during sleep.

Most were concentrated in the period immediately before waking up, although the stroke would probably have begun earlier.

Joe Korner, of the Stroke Association said: "Previous studies have shown that stroke risk does vary over the 24 hour cycle and that occurrence during sleep is most common for ischemic strokes. "This new study confirms that finding. However, more information is required about the different subtypes of ischemic stroke -- there are several different types, each with very different causes."