As a general rule of thumb that applies to healthy individuals:
A healthy sleep (for a monophasic sleeper) is 7-8 hours long and consists of 5 sleep cycles where the first lasts for 70-100 minutes and the consequent cycles get longer but lighter. Each cycle consists of 4 stages lasting usually 5-15 minutes. Stage 1 and 2 are considered light sleep and this is the best time to be woken up in the morning.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org//sleep/how_to_read_sleep_graphs.html