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.