When we’re not alert,
we’re tired and we’re
going to sleep.
And so, sleep is really
important in terms of
managing our level of A, consciousness
and our cognitive ability.
We know that if you aren’t
getting good sleep and if
you aren’t getting the
appropriate amount of sleep,
this is going to really impact your cognitive
ability and your ability to function.
So, stages of sleep are
clearly distinguished and
studied using a sleep
test or a polysomnogram.
So, again, this is something
that’s near and dear
to my heart because I’ve
spent a lot of time
working in a sleep disorder clinic and we
work with patients and we have them come in
and they come for sleep studies, where
they spend the night and we connect.
We connect them with a
whole bunch of electrodes.
And on this figure, you can see some of the
places that you’d place the electrodes
so you’d have your EEG, which measures
brain activity or brain waves,
then you have the EOG
This measures eye movements.
And then we also have the EMG, which
measures muscle tension in your face.
So what we’ll do is hook
these people up and it
allows us to understand a
lot of different things.
It allows us to understand
what stage of sleep we’re in,
and we’re going to go through
those in just a moment.
And it’s going to allow
us to understand
the amount of brain
activity that’s happening.
So a PSG or a polysomnogram
is a multimodal technique
used to measure physiological
processes during sleep.
So I just highlighted a couple of
the different modes of information
and one was the EEG, which
is electrical activity,
EMG is skeletal muscle movements, and
then we have EOG, which is eye movements.
So multimodal refers to different
modes or types of information,
and we use that to measure
physiological activity during sleep.
a PSG or polysomnographic
test research has shown
the presence of four
stages of sleep in humans.
So we know that there are four stages
of sleep and something called REM.
So non-REM sleep is
stages one through three
or also known as N1, N2, N3.
And then there’s REM, which is
the Rapid Eye Movement stage.
So, you might be looking flipping through
some old textbooks or even online
and you’ll sometimes find that they
have stages one through for plus REM.
So, the more current version of the sleep
phases is one through three and REM.
Now, REM is defined is Rapid Eye
Movement stage and as the name implies,
during this stage of sleep, your eyes
move around a lot and extremely active.
It’s kind of freaky if you’ve never seen
it before but really, really interesting.
So the next time, you
know, your roommates were
asleep or your girlfriend or
boyfriend or parents, sneak in
and take a look and watch them.
And if they’re in the
REM stage, and this is
the stage where we do a
lot of our dreaming,
you’ll notice that their eyes
are closed, but their eyeballs
underneath their eyelids are
moving around frantically.
And it’s because
they’re actually --
it’s quite activated and their brain
is very active in that stage.
So we’ll take a look at each stage
and then you’ll see what I mean.
So stage one (N1)
is light sleep.
You’re feeling sleepy, drowsy but
you’re not actually in a deep sleep.
So this is right when you literally lay
down and, you know, you have your, you
know, Mickey Mouse pajamas on and you’re
all comfortable and you’re lying there
and you’re just starting
to get in that relaxed,
I’m starting to feel sleepy,
getting the heavy eyelids stage.
So it’s a transition between
sleep and wakefulness.
The muscles are active, the eyes roll
slowly, but the eyes could still be open.
Now, if we’re looking
at brain activity,
we notice alpha waves and this is a specific
frequency of eight to thirteen hertz
when you’re awake, to theta waves,
which is three to seven hertz.
So this represents a small portion
of your sleep, so only 5% to 10%
because you’re just
starting to get to sleep.
So in terms of the MCAT, you
should be very familiar
with the different stages of
sleep and my advice would
be you should be also familiar
with the types of brain
wave activity that you see
in that stage of sleep.
So looking at it in
terms of the EEG,
these little squiggly
the type of brain wave
activity that you’re seeing.
So what do you notice here?
So, stage one theta waves look
like they’re getting smaller and you
can see the smaller level of activity.
Now, if we get to
stage two (N2),
we continue to become
And this represents a larger chunk of
your sleep so, almost half of your sleep.
And what’s happening here is
you’re seeing two characteristic
features that let us know that
you are entering stage two.
So remember, stage one
is the heavy eyelids.
Stage two is that you are
now can be considered sleep
because you’re no
longer eyes are open
and you start to see these
two characteristic features,
the K-complex and
The K-complex is that dramatic
peak and valley and it has a
duration of about half a second
and it’s quite large and slow.
So your brain wave activity is little scribbles
and all of a sudden you’ll have this
rise and fall and that lasts, you know, a
couple of seconds -- sorry, half a second,
and then you have the sleep spindle,
which are bursts of activity
and around the frequency
of 12 to 14 hertz.
So these are key characteristics
for a stage two sleeper.
Very little eye movement at this point
and very moderate EMG activity.
So EMG refers to the muscle
tension in your face.
So if you’re asleep and
you’re in this stage,
you’re going to notice that your eyes
are not moving, so you’re not in REM,
and you’re going to have
this kind of droopy --
droopy face. Not a lot of
muscle activity happening.
How does that look?
So you can see here, we’re seeing the
spindles, we’re seeing the K-complex,
and the activity,
again, is quite muted.
Now, if we’re looking at stage three,
this is the deep sleep now.
You’re really in deep. At this
point, you’re pretty much out.
This represents about a
quarter of your night
and you start to see the delta waves, which
are high amplitude and low frequency.
So about a half to three hertz.
Previously divided, this is the
stage that was at three and four.
They’ve now just scooped
that all into stage three,
very little eye movement at this
point, moderate EMG activity,
and this the most -- one of the
most restful stages of sleep.
So if you’re balancing around
one and two, one and two
and you’re not getting a
lot of stage three sleep,
you will still wake up
extremely tired because this
is where the restful
nature of sleep kicks in.
So in terms of
activity, you can see
the brain activity has changed dramatically
versus stage one and stage two.
So things have
really slowed down.
probably the start of the
show for at least for me
is the REM stage and
this is the dream state.
So this is characterized by the rapid
eye movements, thus, the name.
And you start to see a
change in your EEG activity
and it actually resembles the beta waves
seen in individuals who are awake.
So you’re actually
asleep, in deep sleep,
yet your brain is acting
like it’s actually awake.
So it’s not a coincidence
because dreams are happening,
that’s a very active
time in your brain,
and so, that’s being reflected
in this EEG activity.
The REM waves are sawtooth in shape and low
intensity and have variable frequency.
We’ll take a look at the
image in just a sec.
Almost no EMG activity, there’s
complete paralysis in your face.
This is the time in your
sleep cycle when you wake
and you have all that drool
action on your pillow.
When you wake up and your pillow is all
soaking wet, that’s actually a good thing.
That means you’ve
achieved REM -- REM sleep
and you’ve got drool all over your
face, your pillow, your pajamas.
That’s a good night.
Now, it’s termed paradoxical sleep because
physiologically speaking you seem awake.
Your brain is quite active.
But, there’s absolutely
no muscle movement.
And this is evolution and this
is us being self-protective
because imagine if
you had a dream,
and in this dream, you
actually acted out your dream
and you were moving around
and running around.
So if in your dream you’re being chased by
wild dogs and you’re running in your dream,
you really wouldn’t want that
in bed, where you’re moving
around and you’re kicking
and you’re running, right?
So your brain and
body has evolved
and so that it says, “Well,
once I enter the REM state,
I’m actually going to
create a complete paralysis
and I will be completely done so I can’t
move and I can’t act on my dreams.”
So you’re completely out, but
your brain is extremely active.
So, it’s a paradox; brain is awake, the
rest of the body is completely kaput.
So if you take a look
at the brain activity,
you can notice that the REM sleep
is very, very similar to what you
would see in an active state
compared to, say, stage three,
where it’s very slow and it’s
very, very high amplitude.
So individuals will pass through the
different stages of sleep in sequence.
So they actually start in N1,
move to N2 and then go
on to N3, and then --
so in terms of if we’re
looking at the sleep stage,
then we’re looking
at hours of sleep.
So as the night progresses,
what do you notice?
Two things you should notice; first,
that you have multiple episodes of REM.
That’s the first thing.
And the second thing is what?
Look at the duration of REM.
Is it getting shorter or longer?
It’s getting longer.
So what happens is as
your night goes on,
your periods of REM increase so the
length of your dreams increase,
which is partly why when you
wake up in the morning,
you tend to remember the last
couple of dreams you had
and not the one that you had in
the beginning of your night.
So a typical sleep cycle is
considered roughly 90 minutes
which explains why you might have these
awakenings throughout the night,
because you cycle through
very, very light sleep,
stage two, stage three where you’re in
really deep, then you have your dream.
And when your dream is complete,
you kind of get kicked back out
and you go back to stage one
and that is when
you might wake up.
You might toss and turn a little bit,
you might want to get a glass of water,
because that’s the closest point
you’ll be in the night to being awake,
and that happens a
couple of times a night.
So this cycling is repeated
throughout the night,
which is why you have
your multiple dreams,
which is why you move around a couple
of times throughout the night.
So as the night progresses,
time in non-REM decreases and
episodes of REM increase.
So, it’s an inverse
So individuals, you might
ask yourself this.
Do you remember your dreams?
Do you remember the dream
that you had last night?
Hopefully, I was in
your dreams, right?
So if you aren’t
remembering your dreams,
that might be an indicator that
you’re actually not achieving REM.
And not to alarm you, but that’s
not necessarily a good thing.
You’re going to want
to achieve REM.
But you can have, you know, periods
of REM that are short or you might
not achieve lots of episodes of
REM and still have some REM,
but these are things that are
indicators of how rested you are.
And how do you feel when you
wake up in the morning?
Do you feel really rested?
Or are you saying, “I still get my
eight hours but I’m exhausted”?
So there might be some things that are
preventing you from getting into REM.