human sleep cycle

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Humans are strange creatures, displaying a close enough to infinite variability in most things. Therefore, we often use the handle of averages to describe them. Human sleeping patterns and characterstics are quite variable, but general trends and commonalities can be identified. The often mentioned average required sleep is 8 hours. However, many people require as little as 5 hours, whilst others require up to 12 hours, to be a fully functioning little joy-bud. Most people can chronically restrict their sleep by one hour from their optimal amount without noticeable effects beyond occasional tiredness. A two hour restriction on a regular basis is more than most people can handle.

Sleep across the night:

Generally, once the time of sleep onset latency (SOL) has passed (you count sheep during this time perhaps), you start the sleep cycle proper. Progressing through SWS slow wave sleep stages 1 and 2, your first block of SWS will be fairly shallow, and you will often come close to waking at the end of it (SWS 1 is often a semi-wakeful state during which you can respond to external stimuli). Note that SWS is a subset or alternate naming convention for NREM sleep, basically, anything that is 'non-REM' sleep.

As the night progresses, the blocks of SWS will become longer, and deeper (more Delta activity will be seen). SWS stage 4 is said to be the deepest sleep, as during this time you are least responsive to stimuli, and physiologically, not doing much at all beyond staying alive. Certain neurochemical and hormonal emission/production cycles are attached, or at least, covary with this cycle, but that is detail for another node, or noder. You will wake up, or come very close to it, many times during the night. You usually don't remember these periods of wakefulness come morning.

Between the blocks of SWS, REM sleep begins to appear. This is the dreamers sleep (you can dream in SWS, but much less frequently, and of a somewhat different nature). It is named for the rapid eye movements that characterise it, which have actually been tied to eye movements played out in dreams (via studies of lucid dreams). During this type of sleep, your body is pretty much in a state of paralysis. Your brain's activity during REM sleep is quite similar to that seen while you are awake. REM sleep is also known as paradoxical sleep, since it is so similar to waking in terms of brain activity, yet the physical state of the sleeper is quite distinct from wakefulness. As the sleep period lengthens, the blocks of REM become longer, and the SWS blocks shorter and more shallow (more likely to just go to SWS 3 than 4).

So, first half of the night - much SWS. Second half of the night - SWS and much REM. However, you can't just go to bed really late to jump straight into the dreamy REM, as the cycle to a large degree needs to follow the SWS to REM transition process. If you go to bed really late, you'll just get the SWS, then your alarm will buzz and it will be time to go to work without any bizarre chicken overlord dreams to kill the pain.

Sleep during the day:

If you oversleep, the REM periods become more intense, and longer. We appear to possess a propensity for particular types of sleep that are linked to time of day, leading some to propose that the stage cycling of sleep follows a more basic ultradian rhythm not tied to sleep alone. Morning naps are often associated with big chunks of REM sleep. By the afternoon, you are ready for some serious SWS again. Afternoon naps are choc-a-block full of SWS 4 goodness. Some have gone as far to propose that an hour of afternoon sleep is worth twice the time of sleep at night, but any type of nap is most beneficial if you do it on a regular basis, not as a sporadic catch-up strategy.

Effects of sleep loss:

With sleep deprivation, you will experience rebound effects designed to catch you up on the type of sleep which you have missed. Generally, the sleep debt is first paid for SWS, then REM sleep if time permits. If sleep is chronically restricted, REM sleep generally is the stage which pays the price. So if you want to keep adding to your dream log, either nap in the morning, or get thee to bed early at night. Paying back sleep debt can occur over a period of days. Paying back a heavy debt is often associated with waking up with a feeling that you may very well have died during the night, and some kind soul reanimated you as a zombie (sleep inertia). Put this one down to your body/brain pumping you full of your own sedatives to ensure some extra deep sleep action.

The terms in brief:

In environments in which they cannot perceive day nor night, humans do not typically follow the 24-hour sleep schedule. The actual schedule followed by humans is dependent upon the sleep habits of the individual.

Velikovsky first declared that the human sleep cycle is in exact accordance with the Martian day of 23 1/2 hours, rather than the Earth day. However, Velikovsky is perhaps not one of the most credible scientists ever to speculate on the subject (though perhaps one of the most imaginative). Later studies concluded that the sleep cycles of humans more often range from 24-28 hours, meaning that people in seclusion will frequently go to sleep as much as four hours later each night, depending again upon the individual.

It is speculated by many that the tendency toward a longer sleep cycle is related to the individual's rate of metabolism.