Your Sleep Cycle Revealed
What's going on in your brain while you're catching your zzz's each night.
Posted Jul 26, 2013
In my last post, I asked "are you getting enough sleep?" Today, in an effort to better understand the role of sleep in our lives, I ask 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.
What does all of this mean for napping? Giving yourself a full sleep cycle (90 minutes) can help you retain certain skill you’ve just learned, but for recovering from fatigue a 15-20 minute nap is ideal (and some research suggests a nap as short as 5 minutes could be beneficial!), since the farther along you are in your sleep cycle, the harder it is to get over that grogginess you sometimes feel when you first wake up (known in the sleep world as sleep inertia).
Would you prefer to be on a biphasic sleep schedule (6 hours at night, 90 minutes in the afternoon)? Do you have some other sleep cycle? Do you often nap? For how long?