Hello everyone, my name is Jared Kim.
I’m a Human Biology Major undergraduate at UC San Diego
and part of the BioClock Studio Winter 2016.
Today, I will be talking about the
2-Process Model of Sleep Regulation.
While scientists are still figuring out the full function of sleep,
we do know that lack of sleep influences people’s daily lives negatively
and leads to some serious consequences,
like a loss in attention span, memory issues,
and overall impaired brain activity
as well as increased risk of obesity.
Sleep in its simplest form is controlled by two processes:
the circadian clock or Process C,
and sleep homeostasis or Process S.
One component, the circadian clock, is an internal daily biological clock
present in humans and many other organisms.
The word circadian comes from the Latin meaning “about a day.”
Thus our circadian rhythms are 24-hour rhythms.
In humans the circadian clock controls physiological processes
that rise and fall every 24 hours, with a model example of this being
oscillations in body temperature.
As you can see the body temperature peaks around 5-6PM
and is lowest around 5-6AM.
This cycle then repeats every 24 hours.
Process C is controlled by the same circadian clock that regulates body temperature.
This graph plots the circadian alerting signal,
which controls how alert the person feels,
with its peak around 6-8PM.
Conversely, the person would feel least alert around 4-6AM.
This rhythm repeats itself from one day to the next
and is distinct from a sense of sleepiness that is
related to how long a person has been awake.
Process C explains why you can get a “second wind” even when you are sleep-deprived.
Which brings us to the second process involved in sleep-wake cycles called sleep homeostasis,
also known as Process S.
Unlike the circadian clock, which re-initiates a cycle on its own every 24 hr just like an
oscillator,
Process S is more like an hourglass that requires an event to restart it.
A person’s sleep pressure will continuously build up as long as the person is awake.
Once the person falls asleep, the sleep pressure dissipates just like turning that hourglass
over and letting the sand flow in the other direction.
As you can see in this graph, when the person wakes up at 9AM, the sleep pressure is very
low,
because this person slept long enough to deplete all the sleep pressure from the night before
and just woke up.
But the longer they stay awake, the higher the sleep drive builds until this person eventually
falls asleep, in this case, around midnight.
Of course, remember that sleep drive is working against the circadian alerting signal which
is why even as we begin to get sleepy in the evening from buildup of sleep pressure, we
can still stay awake.
With these two processes we see how a normal sleep-wake cycle is formed, assuming the person
sleeps around 12AM to 9AM.
The circadian clock provides a daily cycle of alerting signal, which encourages wakefulness
during the day, and sleepiness during the night.
Working against the circadian alerting signal is the homeostatic sleep pressure, which builds
up while the individual is awake and is alleviated when the person sleeps as shown here as blue
arrows.
The sleep pressure antagonizes the circadian alerting signals and the balance of circadian
alerting signals and homeostatic sleep pressure creates a normal waveform of wake propensity
alertness.
As you can see, this person follows a regular sleep pattern with their alertness wake and
sleep episodes during the same interval of each 24-hour period.
This person follows a normal pattern, sleeping in the first part of the night because homeostatic
sleep pressure is high and staying asleep in the second part of the night due to a low
circadian alerting signal.
Conversely, during the day the person is awake in the morning because there is low homeostatic
sleep pressure, and staying awake later in the day even the sleep pressure has built
up because the circadian alerting signal is also high.
So now that we know how a normal sleep-wake cycle is formed, what happens when we put
stress on our body and don’t sleep for a night?
Whether it’s pulling an all nighter, staying awake for the night shift, or binge-watching
your favorite TV show, not sleeping will have adverse affects on your normal sleep-wake
cycle.
As you can see here, this individual decided to skip sleep on the second night.
Sleep homeostasis continues to build up sleep pressure due to the lack of sleep continuing
until the third night where the individual finally sleeps.
The circadian clock continues to cycle normally and provides a consistent alerting signal
interval.
With this huge rise in sleep pressure the person’s alertness level will fall greatly
during the time where the alerting signal is very low, this leads to a high risk of
falling asleep [point].
In addition to this risk, this person accrues a sleep debt, and just like a financial debt,
this has to be paid back.
The problem is that just sleeping your normal amount the next night is not enough to make
up for that lost night.
You have to sleep longer than your normal amount to make it up for several nights to
pay out your “sleep debt”
That’s it for this video on the 2-process model of sleep regulation.
For more information on circadian biology please visit the UC San Diego BioClock Studio
by clicking the link.