Sleep, Dreams and Biorhythms

We sleep in cycles.
Each cycle is 90 to 120 minutes.
There are 3  NREM (N1, N2 and N3) and 1 REM makes a cycle.

N2 is Light Sleep

N3 is Deep Sleep

Memories are probably stored during deep sleep in a raw and random fashion.

NREM        =      Non Rapid Eye Movement

REM           =      Rapid Eye Movement

SCN          =      Suprachiasmatic nucleus
                          Regulates Diurnal Rhythm

In REM sleep memory is reorganized and redundant information is filtered off.

REM sleep is generally 25% of the total sleep and we tend to dream in REM sleep.
Memory is stored during REM.
REM starts as short bursts and as one sleeps longer REM tends last longer.

We have 6 cycle per night.

We generally needs 8 hours of sleep out of which 4 hours of Deep Sleep is vital.

One need not worry about  since these patterns are determined by melatonin secretion.