Tag Archives: sleep

Sleep debt

Image credit: Henry Meynell Rheam , via Wikimedia Commons For students across the world, and especially at Berkeley, sleep seems to be a commodity that is scarce. I know that many of us are willing to risk hygiene, public embarrassment, as well as a healthy social life for the chance to eliminate a couple
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The sleep cycle: what’s really going on while you’re catching your zzz’s

Today I continue on my quest to understand sleep and its role in our lives by attempting to answer a very basic question: what happens when our heads hit the pillow at night?

The sleep cycle: A sleep cycle lasts about 90 minutes and during that time we move through five stages of sleep. The first four stages make up our non-rapid eye movement (NREM) sleep, and the fifth stage is when rapid eye movement (REM) sleep occurs.

NREM sleep: Across these four stages we move from very light sleep during Stage 1 down to very deep sleep in Stage 4. It is very difficult to wake someone who is in Stage 4 sleep. Across NREM sleep, we have little muscle activity and our eyes do not typically move, but all of our muscles retain their ability to function.

REM sleep: As the name would imply – during this final stage of sleep, we have bursts of rapid eye movements. This is the stage of sleep in which most dreaming occurs. Our eyes are not constantly moving, but they do dart back and forth, up and down. These eye movements may be related to visual images of dreams, but that is not confirmed, and in general, the reason for these eye movements is still a mystery. Although our eyes are moving rapidly, the muscles that move our bodies are paralyzed (other important muscles, such as our heart and diaphragm continue to function normally).

So what happens over the course of a night of typical sleep?
It turns out it is not as simple as putting together 4 to 6 of the 90 minute sleep cycles I described above. Over the course of the night, the amount of time we spend in a particular stage of sleep begins to shift. As shown in the figure below, during the first 2-3 sleep cycles, we spend most of our time in deep NREM sleep (stages 3-4), whereas during the final 2-3 sleep cycles, we spend much more time in REM sleep accompanied by lighter NREM sleep. And the complexity of sleep doesn’t end there – apparently how much NREM and REM sleep we get is not just based on where we are in our nightly sleep, but it also depends on what time of day (or night) it is. Regardless of when you fall asleep, people tend to experience more NREM sleep in the earlier hours of the night (e.g., 11p – 3a) and more REM sleep in the later hours of the night (e.g., 3a – 7 a). So those after-hours mutants are getting more REM sleep overall than are the early-to-bedders. As with many other aspects of sleep, the need for all this complexity in our sleep cycles is still a mystery.
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