What is sleep?
The state of consciousness which we call sleep is, for the most part, instrumentally determined, using what is called and electroencephalogram (EEG). The EEG is a summation of the activity of post-synaptic cerebral neurons (Carlson), so it's kind of a measurement of brain activity-not only measuring the frequency of neuronal firing, but the amplitude of firings, as well. A distinguishing factor between the waking state and the various levels of the sleeping state comes from observing and recording the different frequencies/amplitudes of our neuronal firing.

While we are awake, we have two basic patterns of EEG activity, Alpha Activity and Beta Activity. Alpha activity coincides with daydreaming or meditative behavior and is more pronounced when the eyes are closed. Alpha waves are characteristically very synchronized and have pulses of 8-12 Hz. Beta activity represents our attentive/actively thinking state of mind and shows up as irregular, desynchronized, low amplitude pulses of 13-30 Hz. The way I like to think about it is that the harder we need to think, the more frequently our neurons have to fire.

As we enter into sleep, EEG patterns tend to become more synchronized, the frequency of firing slows, but the amplitude of those pulses increases.

The Stages of Sleep
Stage 1 of our sleep cycle is characterized by Theta Activity, pulses of 3.5 to 7.5 Hz. This is the transitional period between sleep and wakefulness. After about 10 minutes, we move on to the next stage.

Stage 2 also shows the Theta Activity of stage 1, but also contains "sleep spindles"(small bursts of 12-14 Hz waves that occur between 2 and 5 times a minute throughout stages 1-4 of the sleep cycle) and "K-Complexes"(sudden, sharp, high amplitude waves which are unique to stage 2 sleep). If a person is forced awake at this stage, they will often deny that they've been sleeping. After about 15 min. we move into the third stage of the sleep cycle.

Stage 3 is characterized by high amplitude Delta waves, which have a frequency below 3.5 Hz, mixed in with the Theta activity of stages 1 and 2. There is a easy, ill-defined transition between stages 3 and 4.

Stage 4 is signaled by greater that 50% Delta Activity and typically lasts for about 45 minutes in a healthy individual. This is the deepest part of our sleep cycle and is the stage in which nightmares typically come out of the closet to visit the hapless sleeper.

Stage 5 is better known as REM sleep and typically begins about 90 minutes after the onset of sleep. REM sleep is associated with a desynchronicity in EEG readings reminiscent of the waking state, and has Theta waves as in stage 1 of the sleep cycle. A person typically carries out storyline type dreaming in this stage of the sleep cycle(storyline dreams differ from nightmares in that nightmares often are just a single, fixed and oppressive situation that the sleeper experiences whereas a dream often runs like a movie) and is very easily waken up. Interestingly, except for the eyes and diaphragm, skeletal muscle tone is lost, essentially paralyzing us while our eyes dart back and forth beneath our eye lids.

That, then, is a summary of the 5 stages of the sleep cycle. Stages 1-4 are commonly called the slow wave, or non-REM stages and occur mostly in the first half of the night, which brings up another interesting little bit of information. As the night progresses, we fail to sink as far down in sleep as we did in the previous sleep cycle such that the time we spend in stages 3 and 4 are reduced while the time, or frequency in which we visit REM increases(Carlson).

In my very uneducated opinion, this is why we can get by with just a few hours of sleep. The majority of the really deep sleeping comes in the first half of the night, so by getting 4 hours or so, we can still extract enough of the restorative benefits that deep sleep gives us. There is a lot of literature available on how various conditions or chemicals effect the amount of time we spend in either Slow-Wave or REM sleep, and I hope future essays will delve more fully into these subjects. Luckily for researchers studying the subject, the origins in the brain responsible for sleep are still not fully understood and thus, facilitate continual probes into how different parts of the brain are behaving during sleep and how to influence those parts to give us the best or worst possible night of sleep. In the next section, I will look at a couple of hypotheses on:

Why We Sleep
There are two major hypothesis for why we sleep. The first, is that it is an adaptive response so as to keep us from stumbling around in the dark. The second theory, is that sleep provides a restorative process(Carlson). Let me first examine sleep as an adaptive response.

The interesting thing about sleep is that it leaves us in a very vulnerable state. In fact, it has been shown that those species that are commonly preyed upon, sleep very little. They must be alert to avoid the perils of the night (or day, as the case may be). An interesting adaptation that has occurred in such animals, is that sleep may consist of a series of very short naps. The Indus dolphin, for example will sleep nearly 7 hours, but take that seven hours in a series of 4-60 second naps(Carlson). Even more interesting is the bottle nosed dolphin, which sleeps only in one hemisphere of its brain, at any one time. Thus, it is always at least half conscious to alert itself to dangers. For us humans, Webb (sorry, I've lost his first name) suggests that sleep is a way for us to prevent bumbling around the dark, avoiding its hidden dangers. Webb suggests sleep is a behavior we engage in when there is nothing else important to do (Carlson). I am not too sure of this logic, myself. It would seem to me that we would have evolved such that every part of our day was in someway beneficial to our survival-satisfying hunger, healing, sexual needs, shelter, etc. Which brings us to the second theory. That there is something innately necessary in sleep. Perhaps, that sleep is a restorative process.

In experiments revolving around why we need to sleep, the evidence indicates that our muscles don't necessarily need sleep but, rather, our brain which controls our movements and thoughts needs sleep to function normally. Sleep deprivation studies, however, have been somewhat inconclusive as to what exactly the role of sleep is. The initial problem with these experiments, which were done in the 20's by Kleitman (another guy with no first name), was that it was impossible to have fully rested, sleep deprived subjects. That is, it was impossible for the sleep subjects to stay awake while being inactive. Thus, subjects had to be rest deprived as well as sleep deprived. For instance, rats were put on tables that would tilt whenever the rats started to get sleepy. They would scramble up, the table leveling until the next bout of drowsiness.

With some of the inherent problems involved in doing sleep deprivation studies, let's take a look at what happens when we go without sleep. A study by Horne et al, showed that there was not any physiological stress or inability to exercise in sleep deprived patients. However, it has been found that after a few days of deprivation, problems in perceptions develop. I, myself, can attest to this. Last summer I was working roughly 18 hour days, but never in a linear fashion. So, my sleeping consisted of two or three, 2-3 hour naps and if I went out with friends, maybe only one of these naps was possible. After a few days of this I would often see little black dots floating over the walls of my apartment, and I had trouble with what day of the week it was. In further evidence against sleep being necessary to heal the muscular system, Adey, Bors and Porter showed that quadriplegics have only the slightest of reductions in there slow wave sleep-the deepest part of our sleep cycle. However, Horne has shown that when exercise takes place in warm, humid climates slow wave sleep is increased by 25%(Carlson). Thus, there seems to be a correlation between the temperature of the brain and the amount of slow wave sleep observed. I thought that it would be an interesting experiment to place a group of people in a sauna for a half hour before going to sleep and another group in a walk in cooler for a half hour and monitor each groups sleep cycle characteristics. I would hypothesize that the cooler group would get less of the deep, slow wave sleep merely because of the need to warm the body more-which requires physical movement.