Modern research has found cyclical patterns in our sleep behavior and brainwaves. The repeating pattern seems to last about an hour and a half and it is not unusual for the timing of a nights sleep to be an approximate multiple of 1.5 hours. Each cycle normally has REM (rapid eye movement) and NREM (Non REM) stages with NREM further broken down into 3 or 4 stages. It has also been observed that the restfulness of a nights sleep improves with the number of consecutive cycles completed. In any event we sleep seems to be designed to span the hours of darkness without interruption until daylight. So if one wanted to deprive a sick patient of sleep it is not really necessary to keep them up all night. Occasional disturbances that interfere with the number of completed hour and a half sleep cycles, or otherwise thoughtfully breaks up sleep, may be effective in severely diminishing or destroying the normal restorative properties of sleep without being easily noticeable.
Owner's Manual to the Brain, 2nd
ed, Howard, Section 7.1
Sleep the Gentle Tyrant, Webb