Two-Process Sleep Model

Hi, welcome back. I’m Doctor Cathy Goldstein and this segment we’re gonna talk about the two process model that controls sleep. So why is this important? We really can’t determine what’s negatively impacting our sleep and even strategize how to fix it until we understand how sleep is controlled. So there’s two processes that control sleep. The homeostatic sleep drive, which I like to think of as the hourglass, or sleep hunger. And the circadian rhythm, our internal clock. So the homeostatic sleep drive builds and builds and builds the longer you’re awake, and dissipates with sleep. It’s mediated by neural substances called somnogens, and these substances accumulate with wakefulness, and then they act on the brain to promote sleep.

The somnogen that we know the most about is one called adenosine. Adenosine is interesting, because coffee and other products with caffeine are adenosine receptor antagonists. So these are going to interfere with the effect of adenosine on the brain and make it harder to sleep as well as making you more alert when you need to be. Now If our body just run on the homeostatic sleep drive to control our sleep. We would sleep in multiple bouts rather than 24 hour a day and that’s obviously not a very adaptive way to sleep. So we have our internal clock or circadian rhythm.

Circadian rhythms are the indigenous, inherent near 24 hour processes that allow all organisms on earth to react appropriately at the appropriate time based on the 24 hour solar light dark cycle, that’s due to the earth rotating in on its axis. Their circadian rhythm’s of all different processes throughout the body. But in humans one of the most obvious is the sleep-wake cycle. The circadian rhythm who are at sleep during an environmental night and wake during the environmental day when the sun is out. Where is this internal clock? Well, it’s located in part of our brain called he suprachiasmatic nucleus or SCN. And that’s how our clock ticks genetically,.

The interesting thing about the circadian clock is even though it’s inherent, even though it’s endogenous it’s not a reaction to the light dark cycle it can be modified. And it’s modified by things called circadian time givers, or zeitgebers and the biggest most strong circadian zeitgeber is light. Now the effect of light on the circadian clock is dependent on which time it is received. So if you think about somebody’s natural sleep period, now, this isn’t when you’d like to sleep. This is when your body truly wants to sleep. And in most individuals, that’s gonna be a sleep onset of around 11 PM, and a wake time of around 7 AM or so.

So if you get bright light in the evening time, at the beginning of this habitual sleep period, it’s gonna push the clock later. You’re gonna become more of a night owl. You’re gonna have a harder time falling asleep or waking up. Now, we see the opposite is true with morning light, light that either occurs at the end of the sleep period or immediately after your natural wake time. This makes your internal clock earlier, so you’re more of morning lark. Why is this important? What are all of us doing in the evening time here. We’re being exposed to electronic light, and electronics, handheld electronics like iPad’s, iPhones, they contain blue spectrum light.

That’s what the circadian rhythm is most sensitive to, so what we’re doing is pushing our clock later and later, making it harder to fall asleep and harder to wake up in the morning. Now, in addition to the central clock that is ticking in our SCN genetically, it’s been identified over the last 15 years or so, that there are peripheral clocks all throughout the organs of our body. And these peripheral clocks tell our liver, our pancreas all of our organs what time it is. And that’s why we think people, that sleep and wake, out of line with their clock. For example, people, who work the night shift, maybe had increased risk for disorders, like diabetes, obesity and heart disease.

Because their peripheral o’clocks are out of sync with there sleep-wake cycle, the light dark cycle and that central clock. Now, we talked about this is a two process model. So how does this model interact to control sleep and wakefulness. So if you see this picture here we have a timeline, starting at 8 AM and ending at 8 AM the next day. The top portion of this graph is your homeostatic sleep drive, and the bottom portion of this graph is your circadian alerting signal. Let’s start when you wake up at 8 AM, okay? You’ve slept all night, ideally, so your homeostatic sleep drive is low. But it starts growing and growing and growing throughout the day.

However, I know I, and I’m sure you wanna stay awake until sundown. We wanna participate in daily life and humans were built to live in a world with electric lightning and that’s likely why we have this process. So we need something that doesn’t allow us to fall asleep in the middle of the day after that homeostatic sleep drive grows. And what that is, is the circadian alerting system, which you’ll see here on the bottom. And that increases in an oppositional manner to combat that homeostatic sleep drive. Now, I wanna point out a few different things on this graph. There’s a period of time in the afternoon where your homeostatic sleep drive is pretty high.

And your circadian alerting system is growing, but it’s not high enough yet to overcome that homeostatic sleep drive. And that’s why we have this late afternoon deep in alertness. Now the circadian alerting system grows and grows and grows until around 8 or 9 PM in somebody with a normal circadian phase. We call this the forbidden zone for sleep because at this point the circadian alerting system overshoots that homeostatic sleep drive to promote alertness as you can see by this yellow curve here in the center of the diagram. Now at around 9 PM, the circadian alerting signal drive drops.

Melatonin secretion begins as we see here by our dim light melatonin onset, and that melatonin which is a sleepiness promoting hormone, combined with the already high homeostatic sleep drive. Results in a drop in alertness and promotes sleep onset. Now what happens after you fall asleep? Your homeostatic sleep drive is going to drop and become low. So how do you stay asleep? Because that’s a circadian alerting system remains low, reaching it’s absolute minimum along with your core body temperature at around 4 AM in normal individuals. This helps you stay sleep, despite dissipating that homeostatic sleep drive.

Then in the morning your circadian alerting signal begins to grow again and your homeostatic sleep drive is now dissipated, and you’re ready to wake up and start your day. The interaction of these two processes results in 16 hours of consolidated wakefulness and 8 hours of sleep when functioning optimally.