Sleep stages and circadian rhythms | Processing the Environment | MCAT | Khan Academy
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Sleep stages and circadian rhythms | Processing the Environment | MCAT | Khan Academy

Voiceover: Even though
you’re not conscious during sleep, your brain is deceptively active. It goes through multiple cycles with distinct brain patterns, and it’s very important to your ability to perform normal functions
when you’re awake. You have four main stages of sleep which occur in approximately
90 minute cycles during a normal night of sleep. The first three stages are all considered non-rapid eye movement, or non-REM which I’m going to abbreviate as N1, N2, and N3. N1 is the stage between
sleep and wakefulness. This is when your brain
starts producing theta waves. You might experience strange sensations known as hypnagogic hallucinations, just kind of a fun name. These can include hearing or seeing things that aren’t there such as seeing a flash of light, or a lot of people hear
someone calling their name, or a phone ring, or a doorbell, or something like that. Or if you’ve been doing something really repetitive just before bed, then that can recur in
your hypnagogic state. For example, if you’ve
been on a boat all day, you may still feel like you’re on water when you drift off to sleep even if you’re on dry land. Or something that’s actually called the Tretis effect, if you’ve been playing Tetris for a long time right before bed, then you might experience visual images of blocks or something all moving in the same direction kind of like the blocks in the game. Another common feeling during this stage is a feeling of falling. That leads to what’s called hypnic jerks, or those muscle twitches
you sometimes experience as you fall asleep. So that’s N1, our first,
lightest stage of sleep. Then we move into N2 which is a slightly deeper stage of sleep. Although it’s still pretty easy to wake up someone in N1, people in N2 are harder to awaken. We see more theta waves as well as something called sleep spindles and K-complexes. Sleep spindles are these bursts of rapid rhythmic brain activity. There’s a lot we don’t know about the purpose of each sleep stage and the function of these brain waves, but some researchers
think that sleep spindles help inhibit certain cognitive processes or perceptions so that we maintain a tranquil state during sleep. For example, sleep spindles in some parts of the brain are associated with people’s ability to
sleep through loud noises. K-complexes are kind of similar, but they’re a different
type of brain activity that’s also thought to
suppress cortical arousal and keep you asleep basically. They’re also thought to help with sleep-based memory consolidation which is the theory that some memories are transferred into your long-term memory during sleep. What’s cool is that
even though K-complexes do occur naturally, you can also make them occur by gently touching someone who’s in this stage of sleep like just brushing against their skin and that will induce some K-complex activity. What your brain does is sort of assess that the touch on the
skin is non-threatening, and it suppresses the
processing of that stimuli to help keep you asleep. Beyond N2 we have N3. This is our last non-REM stage. N3 is also called slow-wave sleep because as you might guess brain waves are very slow. These are called delta waves. They have a range of about
point five to two hertz. You get basically a
half to two oscillations of these brain waves per second. When you’re in N3 sleep, you are very much dead to the world, and really difficult to wake up. If you walk or talk in your sleep, this is the stage where
those things happen. The last stage we need to talk about is REM sleep. REM is R-E-M, stands
for rapid-eye movement. That’s because this stage is when your eyes move really rapidly beneath your lids. Also, most of your other
muscles are paralyzed. This paralysis might actually be a good thing because most dreaming occurs during REM sleep. If you weren’t paralyzed
your muscles might act out whatever you were dreaming about which could be unsafe for you and anyone sleeping near you depending on what type of dreams you have. REM sleep is sometimes
called paradoxical sleep because your brain
actually seems very active and awake, but your body is prevented from doing anything. It’s kind of crazy. Now in a normal night
of uninterrupted sleep, you cycle through these stages about four or five times each. It takes about 90 minutes to go through a complete cycle, but the order, it doesn’t just go one, two, three, REM. The order within a cycle tends to go from N1 to N2 to N3 and then back to N2 before entering REM sleep, and then back to N1, and then it starts all over again. How long each stage lasts depends partially on how old you are, and partially on how
long you’ve been asleep. You tend to do a lot more slow-wave sleep, or N3 sleep in the first few hours, and then more REM sleep
right before you wake up. That’s why if you really want to try to remember your dreams, you can set your alarm to go off a little earlier than
usual like 15 minutes so you’ll get jerked out of REM sleep and are more likely to
remember what you dreamed. You know that you get
tired probably around the same time every afternoon or evening, and you might wonder how your body knows when to fall asleep, or why a lot of people get
tired in the afternoon. The answer lies in something called our circadian rhythms. Just like our sleep has cyclical stages, so does our wakefulness, and our transition for
wakefulness to sleeping. Circadian rhythms are our
regular bodily rhythms across a 24-hour period
which is also called our internal biological clock. These cycles control our body temperature which rises during the day, and then takes a brief dip in the early afternoon, and then goes up again in the evening, and then falls during the nighttime. So it can control our body temperature, and our sleep cycle, all sorts of things. Daylight is a big queue
for circadian rhythms, and even artificial light can affect your circadian clock. That’s why when you travel somewhere with a big time difference, airplanes will usually adjust the lights in accordance with the time zone of your destination. They’re trying to help you reset your biological clock, but resetting that clock can take time which is when you experience jet lag. Your biological clock says it’s time to go to sleep, but your new time zone
says it’s time to wake up. That transition can be a little taxing if you’re traveling. These circadian rhythms
also change as you age which is why a lot of younger people tend to be night owls, but as they age older people tend to wake up and go to bed early. Your circadian rhythms can also prevent you from sleeping in when you want to sometimes. So maybe you get up every day at six AM Monday through Friday, and then Saturday you
can finally sleep in, but you’ll still wake up around six AM. That’s because your
internal biological clock has adjusted to you waking up at that time so it regulates your metabolism, body temperature, sleep cycles so that you wake up at the same time.

About James Carlton

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61 thoughts on “Sleep stages and circadian rhythms | Processing the Environment | MCAT | Khan Academy

  1. Interesting topic, but it is a strain to listen to this video because of the constant "up-talking". See: for a good explanation. This bad habit is not just with young women anymore; men are doing it too.

    I hope my comment is taken as intended, which is to help produce better learning experience by speaking smoothly allowing the listener to focus on the information, not on the speaker. 

  2. if i'm not wrong also take note in that stage 1 produce Alpha and Theta waves not just Theta, Stage 2 however is just Theta.

  3. how do u writ so good with a mouse? u r the god of writing with a mouse.  I have never seen anyone write so pretty with a mous  how u do it? thx

  4. Every time I have woken up (voluntarily or suddenly due to external stimuli), I have just been dreaming. This has been the case throughout my life. Does that mean I don't get really deep, restorative/regenerative sleep? Can one get really good, restorative sleep while dreaming? Thank you.

  5. I can fall asleep only when I am tired, and it can be anytime during the night,…quite often it it is at different times, so I do not live up to a circadian rhythm, I am completely fine with it.

  6. I keep going Straight to R.E.M. Sleep

    I skip all other stages of sleep.. I have vivid dreams and sleep paralysis even in naps

  7. Hоw I stаrted falling aslеep in 15 minutes oor leeеss =>

  8. Also, the reason why people sometimes don't remember their dreams, is because they awaken between cycles. Everybody dreams, not everybody remembers.

  9. You are very conscious during sleep.. to be unconscious basically is to be in a coma or dead.
    or at least that's what I've been taught in my psychology class.

  10. Interesting. My son sleep walks. Plus 6 and half years ago my boyfriend would wake in the night and try force himself on me. It was scary because he felt so strong. But thing is he never remember doing it

  11. I did not sleep properly until I discovered this awesome bedtime calculator – it works like magic

  12. t took years and years to cure sleep maintenance insomnia. Finally I have found the cure

  13. Talking about you're "k complex" theory I do have dreams that are now in my long term memory as a fact that's forsure.

  14. Your voice is like a bunch of howling raccoons who are all drowning in a river while listening to one of Jake Paul's raps in distorted agony.

  15. REM is a misnomer in that the eyes do not actually move rapidly. They move slower than when you are awake and don’t have to move whatsoever.

  16. your demonstration with graphs sucks! REM sleep produces stage one which is mostly around beta and alpha so basically (same brain wave when someone is also in an awake stage but is in deep sleep/dream) None-REM stage consists of beta and alpha so when enter into REM it produces same brain waves. so it goes non rem beta, alpha , theta, delta and back to (beta) Rem. This consists of repetition of 5 cycles a night maybe consisting of longer beta(Rem) periods every cycle until awake.

  17. so when you wake up in the middle of the night are u in N1 or can you wake up in any cycle? ( if I wake up and then instantly start thinking/over thinking, ((bc of anxiety))

  18. If you say that our body is paralyzed, then how on earth do we breathe ? Is it possible that only parts of our body are paralyzed? Just wondering. Wud love if someone have the explanations:)

  19. Alright. That's wrong data.
    The Textbook of Medical Physiology by Guyton and Hall (pretty much a bible) states that Stage 1 has low voltage alpha spindle bursts and not theta waves.
    Stage 2 and 3 rather has theta waves being produced.

  20. What does the brain release to paralyze the body for sleep? It's fairly easy to trigger this while fully awake.

  21. Has anyone read the light book: how natural and artificial light affect our mood, health and behavior? It has some good sections on sleep.

  22. My cousin beats the shit out of me and shouts in his sleep . I couldn’t sleep when we had a sleep over . Fuck U Omer

  23. Please correct me if I'm wrong. Did you say that K complex helps with memory consolidation?
    My book says that sleep spindles are the ones that help with memory consolidation. who is wrong? or if none is, can you please explain why the book says one thing and you say something different? Thank you and great job on explaining.

  24. I gotta get up at 6 am. Been doing it for 12 years. Now, when I wanna sleep in, like the video says, my eyes snap open at 6 every day. At least I never miss the bus.

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