The theory behind it is that, yes, you will wake up more refreshed with three hours of sleep than four, because if you wake up mid-cycle you will be terribly gr

The theory behind it is that, yes, you will wake up more refreshed with three hours of sleep than four, because if you wake up mid-cycle you will be terribly groggy (a problem I suffer from fairly often). The large range of sleep (four times) is designed to matchup with most peoples' typical length of sleep (six to ten hours). As far as differences in sleep cycles, you are totally correct. My research led me to determine that ninety minutes is generally accepted as the total length of four stages of sleep, but any variance from that will skew the results. That said, the vast majority of people (supposedly) complete the four stages of sleep in about 90 minutes.