Every person, from his or her own experience, knows that sleep changes through the years. This is because specific groups of neurons (brain cells) have to develop to be able to perform sleep functions properly. A newborn’s “sleep neurons” are not fully developed, thus neither is their alertness nor is sleep performed in the same way as it is in adults (as parents well know!).
In newborns, sleep is still divided into REM and NREM phases, but the subphases of NREM are only developed between the 2nd and 6th month. What is also very different from adults is that the REM phase lasts more than half of sleep time, nearly twice as much as an adult. REM time diminishes over the years and reaches its final duration when the child is 2-3 years old.
As we grow older, sleep changes from regular periods interspersed by wakefulness to a more unitary form. Starting from the second decade of our life, our sleep begins to age. We are mainly losing the 3rd and 4th NREM subphases, which can be explained by the falling density of neural synapses (contact spots).
From the neurological point of view there is no single sleep system of the brain. There are multiple, non-specific neuron sets which are cooperating anatomically and physiologically with the group of neurons of the so called “parasympathetic system”, which is responsible for e.g. example, lowering blood pressure, slowing heart rate or narrowing the pupils.
As we sleep, both in NREM and REM phases, less blood flows through the parts of the brain which are responsible for association.