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Nocturnal non-invasive ambulatory blood pressure monitoring inevitably causes an undesirable external stimulus due to the cuff pressure and, for some monitors, disturbing compressor sound. The objective of the present study was to determine whether non-invasive automated blood pressure monitoring during sleep provokes arousal and changes in blood pressure and/or heart rate.Sleep response and blood pressure reaction during and immediately after blood pressure monitoring were studied by means of simultaneous electroencephalography and blood pressure recordings.Blood pressure and electroencephalogram were recorded simultaneously in 24 subjects (10 hypertensive, 14 normotensive). Blood pressure was registered with a non-invasive automatic blood pressure monitor every 20 min. Inflation and deflation of the cuff were registered on one of the electroencephalogram channels by means of a small pressure meter attached to the cuff. For each subject, blood pressure during uninterrupted sleep was compared with that during arousal provoked by the recordings.Blood pressure recording caused an arousal in 67% of the recordings. During 33% of the recordings, sleep continued. Neither systolic nor diastolic blood pressure differed significantly for recordings during 'uninterrupted sleep' when compared with arousal. Heart rate was significantly faster during arousal than during uninterrupted sleep. Analysed separately, hypertensive subjects showed an overall blood pressure response close to that of normotensive subjects. There was a significant trend towards lower blood pressure, parallel with deeper sleep.The results of this study show that non-invasive ambulatory blood pressure monitoring during sleep accurately records basal blood pressure and can distinguish blood pressure during superficial sleep from blood pressure during deep sleep. Sleep is often disturbed by blood pressure monitoring but, irrespective of whether recording provokes arousal, monitored blood pressure is the same.