Circadian rhythms may influence when strokes occur. Researchers analyzed data from nearly 13,000 patients who'd experienced one of these three stroke types as a first stroke: cerebral infarction (ischemic stroke, the most common stroke type), intracerebral hemorrhage, and subarachnoid hemorrhage. Dividing the day hours into 12 2-hour segments, the researchers looked for patterns in stroke occurrence. They found that all three stroke types peaked twice during the day: between 6 a.m. and 8 a.m., and between 6 p.m. and 8 p.m.

Strokes were significantly less likely to happen during the night, but about 20% of cerebral infarctions occurred during sleep. Most of these happened immediately before the person awakened.

Because blood pressure drops during sleep, the researchers concluded that sleeping increases the risk of ischemic stroke and reduces the risk of hemorrhagic stroke.

Source: Differences in circadian variation of cerebral infarction, intracerebral hemorrhage and subarachnoid hemorrhage by situation at onset, Journal of Neurology, Neurosurgery and Psychiatry, S Omama, et al., online ahead of print publication, August 17, 2006.