Enhanced slow sleep in extended sleep

The slow wave sleep (0.5-2.5 Hz) of 42 subjects who slept in a laboratory for more than 8 h was examined. Although the amount of slow wave sleep diminished exponentially across the first 11 h of sleep, there is evidence of an increase in amount of slow wave sleep in more extended sleep. This finding complicates the concepts of slow wave sleep as an index of sleep need due to prior wakefulness or an index of a restoration process.