orthogon wrote:That a sleep cycle is about 90 minutes appears to be fairly uncontroversial, but multiplying this up to arrive at the correct duration for a whole night's sleep seems suspect. Presumably there's some variation amongst individuals, and a 6% change in the 90 minutes would turn the nominal 7.5 hours into anything from 7 to 8 hours. Added to that, the hypnogram on the comprehensive Wikipedia page on sleep suggests variation in the length and structure of sleep cycles even during one sleep episode in one individual. So whilst I think we can conclude that 45 minutes is probably a pessimal length of sleep for most people, without taking an EEG it's hard to draw any conclusions for exactly how long we should sleep at night.
The whole topic is absolutely fascinating, anyway.
Fitbit activity monitors are decent for tracking sleep cycles, in my experience. Given maybe a weeks data, it should be pretty obvious how long your cycles are, and give you a pretty good idea of when you should awake. It may not be *exactly* the same every day, but as gm says, a good estimate is enough for general practices.