Most authorities define a normal sleep cycle in about the same way:

1) Sleep is divided into cycles, with each cycle lasting approximately 60-90 minutes. Therefore, we would normally complete about 5-7 cycles in a proper nights sleep.
2) Each sleep cycle has five (5) distinct stages, ending in the REM (Rapid Eye Movement) stage, which is thought to be the most beneficial to good health.
3) Our objective is to complete as many cycles as possible. Although we are interrupted at the end of each cycle, if we can get the complete 60-90 minute cycle complete before we make our first turn or interruption, we are on the way to a restful night.