The point of the calculator is to time your sleep so that you wake up between REM sleep cycle (rather than being awoken by your alarm in the middle of a REM cycle, which usually puts you in groggy zombie mode). The calculator works off the assumption that the average sleep cycle is 90 minutes (1.5 hours), so it calculates times that would allow you to wake up between sleep cycles (9 hours, 7.5 hours, 6 hours, and 4.5 hours all being multiples of 1.5 hours).

https://en.wikipedia.org/wiki/Sleep_cycle