2-Minute Neuroscience: Stages of Sleep
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2-Minute Neuroscience: Stages of Sleep

Welcome to 2 minute neuroscience, where I
explain neuroscience topics in 2 minutes or less. In this installment I will discuss the
stages of sleep. Sleep stages are defined based on the measurement
of electrical activity in the brain using an electroencephalogram, or EEG. An EEG represents
fluctuations in brain electrical activity in voltage as a waveform of variable frequency
and amplitude. The American Academy of Sleep Medicine classifies
sleep as consisting of 4 stages. The first two stages involve drowsiness and light sleep.
When someone begins to fall asleep, they enter stage 1, during which an EEG records low-amplitude
waves of mixed—but mostly high—frequencies. Next, the person enters stage 2 sleep. This
is characterized by the presence of phenomena on an EEG known as sleep spindles and K complexes.
Sleep spindles are trains of high-frequency waves. A K complex involves a biphasic wave
that stands out from the rest of the EEG. Stage 3 sleep is also known as slow-wave sleep
or deep sleep. In stage 3, delta waves, which are low-frequency, high amplitude waves, make
up at least 20% of brain activity. Stage 3 sleep is thought to be especially important
to overall restfulness. Next, the sleeper passes rapidly back through stage 2 and stage
1 before entering rapid eye movement, or REM, sleep. In REM sleep, EEG activity resembles
what’s seen in stage 1 or restful waking. During REM sleep, the muscles are completely
relaxed and limp but the eyes are moving rapidly. This is the time of sleep when our most vivid
dreams are likely to occur. After REM sleep, the person will sometimes
awaken briefly but then will move through the sleep stages again, in order. Most people
will repeat this cycle 4-5 times a night, with each cycle lasting about 90-110 minutes.

13 thoughts on “2-Minute Neuroscience: Stages of Sleep

  1. I work 5 nights a week, and have done for about 12 years. I use (vape) cannabis to help me sleep a solid 6-8 hours everyday, despite temperature and noise levels. I just got a fitbit which tracks my sleep cycles, and suddenly I'm obsessed with the data it produces and am watching all videos on sleep cycles to try to see if mine is normal. I'd always assumed that cannabis mostly kept me in deep sleep, but it turns out I spend most of the day in light sleep, with seemingly random (and short) periods of deep and REM sleep. Sometimes I feel refreshed when I wake up, and sometimes I don't. I never remember any dreams, but that might just be a short-term memory issue.

  2. What about people who jump almost directly to REM sleep. I went to a sleep clinic 20 years ago and I was in REM within 5 minutes each time they flipped off the lights. Back then the docs didn't bother to explain much, I don't even think I was supposed to know what the tech told me.

  3. I have an accurate heart rate and O 2 level monitor to wear when I sleep. How can I estimate my sleep quality using this data? Can you give me an estimate of what happens to my heart rate and O2 level as I move less and less and go into deeper stages of sleep? Thank you for this great video!

  4. I wanted to add a comment to address a common question I get about this video: Although many sources may still refer to sleep as consisting of 4 stages + REM, this approach to sleep staging isn't typically used anymore. Stage 3 and 4 were very similar, and are now usually combined into one stage (stage 3). Groups like the American Academy of Sleep Medicine, for example, consider sleep to consist of stages 1, 2, 3, and REM sleep.

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