[Added January 2005]
Normal sleep consists typically of periods lasting around 90 minutes that repeat 4 or 5 times each night.
Each period starts with a non-REM phase where we
go through 4 stages of sleep until we reach the stage of deep sleep with low
frequency electrical brain waves. Then the REM sleep starts with a lot of brain
activity and rapid eye movements. This ends when we get awake for usually a very short time and then a new cycle of sleep starts: light sleep, deep sleep, REM sleep.
Early experiments have shown that rem sleep doesn’t have any influence
on remembering lists of words or facts(i.e. declarative memory).
Instead rem sleep influences procedural memory (like the ability
to recognize patterns in a computer screen).Motor skills which depend
on procedural memory, are affected also by non-rem deep sleep.
Skills that require visual abilities are improved by both rem and
non-rem deep sleep.Sometimes also an hour with shut-eyes makes a big
It seems that different kinds of memory need different kinds
of sleep.This may explain why sleeping on a problem can produce sometimes
the result of getting awake the next morning with a solution.
Studies in animals have shown that during the sleep the same neurons
activated during the day by doing some task like traversing a maze, are activated again.
It seems as if during the night we are re-enacting (simulating) the same
experiences that we had during the day. This seems to produce two results:
- reinforce some pathways making permanent some memories
- drop memories connected to experiences considered irrelevant
This second task may be the result of the work done during slow waves deep
sleep when connections between neurons seem to be indiscriminately weakened.
So the sleep is perhaps a series of repeated cycles of pruning and strengthening neural connections that let you learn new skills without forgetting old ones.