5. The stages of sleep

As I mentioned previously, there are 4 stages of sleep

Stage	type	A.K.A.	Duration(undisturbed)
Stage 1	NREM	N1	7-8 min
Stage 2	NREM	N2	15-25 min
Stage 3	NREM	N3, slow-wave-sleep(SWS), deep sleep	20-40 min
Stage 4	REM	REM sleep	10-60 min

 Stage 1-3 are NREM, as in non rapid eye movement. Basically this means that only in stage 4 is there rapid movement of the eyes which is caused due to vivid dreaming.

All 4 stages make up one sleep cycle and at least 4 cycles are required per night for proper sleep.

Every cycle on an average takes 90-120 min and this 4 sleep cycles take 6-8 hours to complete.

More information on these stages will be there in further articles. 

Incomplete/ disturbed cycles can lead to tiredness during the day. Chronic(permanent) disruption in the circadian rhythm and the sleep cycle can cause various issues ad disorders. These are called sleep disorders or circadian rhythm disorders.