In my last blog we were reviewing the fundamental question of why we sleep. We left off trying to sort out the level of activity of the brain and body during different stages of sleep (i.e. Non-REM and REM).
During non-REM the body and brain are at their most relaxed state. Ironically, most people think of REM as deep sleep, but actually during REM the level of brain activity resembles the awake state. Our most vivid dreams occur during REM, signified by the highly active brain. In fact, the motor centers that allow us to move when awake are also very active during REM, but their function is inhibited or blocked by hormones in the brain that essentially paralyze the muscles, with the notable exception of the eye muscles- the rapid eye movements of REM are what give this sleep stage its name. (We touched on this idea when we talked about REM behavior disorder, which is a disease state that causes the normal paralysis of REM to be lost and allows one to “act out” dreams.) REM also affects the other organ systems. In REM breathing and heart rate tend to fluctuate a lot, just as they do when you are awake.
I hope that I have been able to convince you that not all sleep is the same, and different things are happening to our body and brain during the different stages of sleep, but we still haven’t approached the question of what function sleep serves.
First let’s point out that sleep is necessary, possibly even more than food. In one well known experiment, rats that were completely sleep deprived died within 10-20 days. This is quicker than if they were totally deprived of food, but allowed to sleep normally. In humans there is an extremely rare genetic disease called fatal familial insomnia that leads to death in a few months, possibly due to inability to sleep.
So now that we have established that sleep is necessary for life, we can explore some of the theories as to the function of sleep.
When body tissues metabolize the fuel that helps them run, they give off waste products, known as free radicals. These free radicals can damage the DNA of a cell, but most organs are unable to deal with this damage by replacing the “broken” cells with newly produced ones. Unfortunately, most areas of the brain are unable to produce many new cells after birth. So it has been believed for some time that the low metabolic state of non-REM sleep gives the brain an opportunity to repair itself, by removing the toxins produced during wake.
Other research studies have shed some more light on what the function of slow wave sleep is. There are two types of memory: declarative and procedural. Declarative memory refers to memory that can be “declared”, like facts and figures, essentially “book knowledge”. Procedural memory concerns the ability to perform skills, like performing motor tasks. It seems that slow wave sleep enhances our ability to remember facts, our declarative memory, while stage 2 sleep (a different portion of non-REM sleep, actually the stage that adults spend the majority of their sleep time in) seems to improve procedural memory.