The sleep in young adults with severe isolated growth hormone deficiency (IGHD) was examined by polysomnography. There was a significant decrease in delta sleep time (= stages 3 + 4, slow wave sleep, 'deep sleep'), especially in stage 4. The total sleep time was significantly increased compared to age- and sex-matched normal subjects. The increase in total sleep time was related to an increase in stage 1 and stage 2 sleep. There was no significant difference in total rapid eye movement (REM) sleep time, but when correcting for the very long sleep time in the IGHD subjects, the cumulated REM time within the first 390 min of sleep was significantly less than in the normal subjects.