A new study in the journal Neurology finds this affects about 14 percent of stroke patients or 58,000 people.
The only treatment for ischemic stroke must be given within a few hours after symptoms begin. But, because people wake up with symptoms, it is difficult to tell when they really started.
Researchers are working on new imaging studies to better identify those who are still eligible for treatment.
In the meantime, doctors say, if you wake up with strange symptoms, don't sit around, seek medical attention immediately.