I'm no expert on this either; I just studied this section of the book pretty thoroughly :) What you say is correct, but about SWS ("Slow Wave Sleep", or stages

I'm no expert on this either; I just studied this section of the book pretty thoroughly :) What you say is correct, but about SWS ("Slow Wave Sleep", or stages 3 and 4 -- stage 1 being when REM occurs). These are the main recouperative stages. Eliminating REM sleep (e.g. by waking someone up at the start of a REM cycle then letting them go back to sleep when the cycle would have ended) has little or no effect on alertness the next day, but doing the same for SWS cycles results in symptoms of sleep deprivation. It has been hypothesized that REM sleep plays a role in strengthening memories, but evidence for this is spotty. Regardless of how many hours you're asleep, reducing the amount of time spent asleep reduces the amount of REM sleep, but not NREM sleep, making you a "more efficient" sleeper. The main evidence for long-term sleep reducton comes from studies by Friedman et al. (1977) and Mullaney et al. (1977). In these studies, which covered eight subjects over a period of several months, two reduced their sleep down to 5.5 hours, four down to 5 hours and the remaining two down to 4.5 hours. Below about 6 hours, the subjects reported daytime sleepiness, which increased as sleep was futher reduced. A follow-up study a year later showed that the subjects were sleeping 7-18 hours less per week. With only eight subjects, I'm a bit concerned about the generalizability of this study, but I think it's good enough for my purposes.