Stroke is a leading causes of death and disability. At least 20% of strokes occur during sleep, so- called 'wake up stroke'. Thrombolysis with the clot-busting drug alteplase is effective for acute ischaemic stroke, provided that it is given within 4.5 hours of symptom onset. Patients with wake-up stroke are currently ineligible for clot-busting therapy. Previous studies indicate that many wake-up strokes occur just before awakening.