CrimePsych

For most of us, sleep is nothing but a compulsory action that we have to do at least once a day, but for scientists who study it, the mysteries of sleep contribute to a plague of questions with hard to decipher, often inaccessible, answers. Questions such as what function does sleep serve and is there an internal mechanism controlling our sleep are among various possible questions that researchers in the field continue to scrutinize but in this particular paper we will examine the stages of sleep, their physiological differences and a number of the theories behind Rapid Eye Movement (REM) sleep.

Sleep is broken into a number of different stages with distinct physiological dissimilarities across them all, aside from one particular stage transfer. These separate stages are characterized and measured by the electrical activity in the brain as determined by an electroencephalograph. The stages of wakefulness are thus broken into two distinct sub-groups consisting of alpha and beta activity; beta activity, distinguished by irregular, mostly low amplitude waves of around 13 – 30Hz occurs when a subject is alert, attentive or actively thinking whilst alpha activity is characterized by regular, medium frequency waves of 8 – 12Hz. This second type of activity usually only occurs when a subject has their eyes closed and is in a resting state.

Upon entering Stage 1 sleep the presence of theta activity begins which is depicted by waves of 3.5 – 7.5Hz. This is actually a transitional stage between wakefulness and sleep with subjects’ eyelids slowly opening and closing, as well as their eyes rolling up and down in their sockets every so often.

After approximately 10 minutes of Stage 1 sleep we enter Stage 2, and again we see a physiological change in brain activity although nothing out of the ordinary. The emergence of sleep spindles (short bursts of 12 – 14Hz activity) and K-complexes (isolated high amplitude waves) occur but for the most part the electrical activity in the brain remains the same with irregular waves and periods of theta activity.

Stage 3 sleep is entered after approximately 15 minutes in Stage 2 and we see the emergence of high amplitude delta activity of less than 3.5Hz. This stage and Stage 4 are referred to as slow wave sleep and are considered to be two of the most important stages of sleep. Researchers theorize that these two stages are the brains time to rest. The physiological changes that so clearly characterized a stage change in previous stages becomes not as clear cut upon entering Stage 4 sleep as they both comprise the same type of activity. Stage 3 has approximately 20 – 50% delta activity, while in Stage 4 this jumps up to greater than 50%.

We now come to the stage that we will focus the remainder of our attention to and that is the Rapid Eye Movement (REM) stage of sleep, often referred to as paradoxical sleep as the physiological activity abruptly changes. The EEG suddenly resembles that of a subject who is awake but with sprinkles of theta activity (seen in Stage 1 sleep). The release of neurotransmitters such as serotonin, norepinephrine and histamine is shut down completely resulting in muscle atonia, a state in which the motor neurons are not stimulated while cerebral blood flow and oxygen consumption accelerate. The penis will also become at least partially erect and females’ loins will moisten independent of sexual arousal or sexual dream content. So, all physiological aspects aside, why do we have REM sleep?

With the mountain of research that has been conducted in the fifty odd years since its discovery in 1953 one would think we would have a few more solutions but we are still lacking in our comprehension. However there are a few promising theories that seem to be receiving the greatest amount of attention from the scientists of sleep. The best avenues of explanation thus far focus on REM sleep as a time for brain development and learning or memory consolidation.

Newborns and infants spend considerably more time in REM sleep than adults and elderly people, the differentiation ranges from more than 80% for newborns down to as little as 10% for people who are elderly and suggests that this is when brain development actually occurs. However two probable hypotheses are born from these observations; that brain development causes REM sleep, or that REM sleep is when brain development actually occurs.

But animals that are born with more developed brains tend to spend less time in REM sleep than those born with underdeveloped brains. So this seems to endorse the hypothesis that development actually transpires during REM sleep rather than brain development is the cause of REM sleep. Commonsense also tends to make one think that the brain needs to be as close to switched off as it can be before any development can effectively occur.

Another widespread theory as to why we have REM sleep is to do with memory consolidation and learning. One hypothesis is that memories of the previous day are consolidated and integrated with existing memories during REM sleep while others tend to think the opposite; that REM sleep is when useless and mundane information is flushed from our memories to avoid clutter. Research also suggests that when animals are deprived of REM sleep they learn a task more slowly. Whatever the case, REM sleep deprivation seems to have a number of effects on cognitive and memory functions that still require more exploration.

There appear to be no easy answers, sleeps very nature makes it extremely difficult to research, as unlike other types of research where one can ask a subject how they feel etc. only inferences can be made as to the reasons behind it. That being said, the main theories we have discussed here have a respectable amount of evidence to support them. However there are likely to be various other theories that will gain force, or even just emerge over the coming years, for example, the suggestion that REM sleep is also the time when the monoamine receptors in the brain can recover to full sensitivity. Until more empirical evidence is proposed, the nature of sleep will remain somewhat metaphysical albeit on the right track.