A sleep cycle typically progresses N1 -> N2 -> N3 -> N4 -> N3 -> N2 -> REM, before repeating.
The first cycle in a night takes less time--at 70 to 100 minutes--and has the longest N3 and N4, and shortest REM interval. As the sleep progresses, the N3 and N4 phases shorten and REM lengthens, as the overall cycle for 2nd and subsequent cycles extends to 90 to 120 minutes. Later cycles may omit N3 and N4 entirely. The only phases in which you are unlikely to awaken from external stimulus are N3 and N4, so by your last cycle of the night, you are more likely to awaken at any time from an alarm clock or a sunbeam on the face.
It may be that N3 and N4 are used to shut down the body, to perform a physical cleanup of the brain and other organs for their metabolic waste products, and then REM is used to clear out short term memory and compress information from it for storage in long term memory. If so, then the body is essentially reserving the first 160-220 minutes in a night for physical cleanup before allowing a significant REM phase to happen. As REM apparently represents as much or greater brain activity as a waking brain, the NREM phases could then clean up the metabolic waste products from the previous REM phase.
If you take the first 3 phases as a double-NREM, middle cycles as REM-NREM, and the final cycle as REM-wake, your time interval math may work out differently.
The first cycle in a night takes less time--at 70 to 100 minutes--and has the longest N3 and N4, and shortest REM interval. As the sleep progresses, the N3 and N4 phases shorten and REM lengthens, as the overall cycle for 2nd and subsequent cycles extends to 90 to 120 minutes. Later cycles may omit N3 and N4 entirely. The only phases in which you are unlikely to awaken from external stimulus are N3 and N4, so by your last cycle of the night, you are more likely to awaken at any time from an alarm clock or a sunbeam on the face.
It may be that N3 and N4 are used to shut down the body, to perform a physical cleanup of the brain and other organs for their metabolic waste products, and then REM is used to clear out short term memory and compress information from it for storage in long term memory. If so, then the body is essentially reserving the first 160-220 minutes in a night for physical cleanup before allowing a significant REM phase to happen. As REM apparently represents as much or greater brain activity as a waking brain, the NREM phases could then clean up the metabolic waste products from the previous REM phase.
If you take the first 3 phases as a double-NREM, middle cycles as REM-NREM, and the final cycle as REM-wake, your time interval math may work out differently.