Basics of Human Sleep

In order to best improve your sleep, it is necessary to first understand the way sleep works. In this section, we will dive into the brain and body to understand the different mechanisms that put you to sleep, keep you asleep, and wake you up in the morning. To start off, take a look at this video from sleep expert, Dr. Matthew Walker, a professor of neuroscience and psychology at the University of California, Berkeley. He introduces the different stages of sleep that are important to understand.

From this video, it’s important to remember the main types of sleep and what they do:

• The three main categories of sleep are non-REM light sleep (stages 1 and 2), non-REM deep sleep (stages 3-4) and REM sleep. These are the essential building blocks of your sleep’s architecture.

• REM sleep is important for regulating and processing emotions, as well as storing new memories and pruning away old ones.

• Non-REM deep sleep is important for the physical restoration of your brain, body, and immune system, among other things.

• Non-REM light sleep is useful for falling asleep and for switching between REM sleep and deep, non-REM sleep. However, when you toss and turn, wake up throughout the night, wake up feeling tired, or feel tired throughout the day, there’s a good chance that you spent too much of the night in the light stages of non-REM sleep.

• During healthy sleep, you typically cycle between REM sleep and non-REM sleep all night long.

• Not getting enough sleep typically results in a disproportionate loss of REM sleep.

Sleep problems are typically caused by an issue in sleep/wake regulation, or in other words, your body’s ability to fall asleep and stay asleep. As such, it would benefit you, the reader, to know how your brain and body put you to sleep. This happens in two main ways: you rely on chemical signals within your body (internal cues) and environmental signals from the world around you (external cues) in order to induce and maintain sleep. 

Internal Cues for Sleep

Your body’s sleep and wake pattern is controlled by two opposing forces: the need for sleep (sleep drive) and the urge to stay awake (wake drive). There are many things that contribute to your sleep drive and wake drive. Here, we will mention four of them. The first one is your circadian rhythm. This “biological clock” not only helps your body determine when to wake and sleep, but also when to eat, use the restroom, and so on (hence why you are hungry at around the same time each day). This clock is so strong that you could be locked in a room without any windows or light, and your body would still know roughly when to wake and when to go back to sleep.

The second internal cue for your sleep drive is a brain chemical called adenosine. This chemical collects in your brain during waking hours. Sort of like the gas in your car but in reverse, adenosine builds up the longer you go during the day. As more and more adenosine builds up in your brain, you begin to feel more sleepy until you have enough to fall asleep. During the night, your brain has a chance to evacuate all of this built up adenosine, a process which typically takes around 8 hours. You can then wake up the next morning feeling refreshed and adenosine free, and the cycle repeats itself. The consequence of not giving your brain enough time to clear out its adenosine is simple: the adenosine will still be there in the morning! This is one reason why not getting enough sleep makes it harder to get out of bed and can make you drowsy throughout the day.

A third internal cue for sleepiness is the hormone melatonin. When light-sensitive cells in your eyes register darkness in your environment, they relay this information to the suprachiasmatic nucleus (SN), a relatively small brain region sits behind the eyes. When the SN gets this signal, it tells the pineal gland (PG) to begin producing melatonin. This "messenger of darkness" goes throughout the rest of the brain and body, letting it know that it's time to sleep. In turn, your body drops in temperature, slows its heartrate, reduces alertness, and transitions into a state of restfulness. The PG continues producing melatonin throughout the night until the SN senses light in the environment again, and melatonin production stops. We sometimes interfere with this process when we deprive ourselves of darkness, causing us to miss out on the regular, sleep-inducing secretion of melatonin in our brains.

For more information on the role of light on sleep, watch this 2.5 minute video.

The fourth internal cue is chronotype. Chronotype tells us why some people seem to be naturally predisposed to fall asleep earlier or later than others. The terms "early bird" and "night owl" aren’t just describing people who choose to wake up early or wake up late. There are actually genetic differences which may predispose us to naturally wake up earlier or later than others. Thus, our genes play a huge role in the timing of our day and when we will be able to best accomplish many different activities, including sleep. Watch this video below (3.5 minutes) and pay attention to the different two main chronotypes listed. Also, recognize what kinds of negative effects could come as a result of an individual not living in harmony with their chronotype. To find out what your chronotype is, take the quiz here.

External Cues for Sleep

Besides the four main internal cues that your body relies on to fall asleep every night, there are certain external cues that also bring on sleepiness. A decrease in social interaction can serve as a valuable cue for sleepiness. When you stop eating during the evening, your body also takes this as a hint that it may be time to go to bed. A drop in room temperature does the same thing because it helps facilitate your body's natural drop in temperature that occurs as you transition into sleep. As we mentioned, the levels of light in your environment is also a crucial external cue for sleepiness. Last of all, lower levels of noise signal to the brain and body that it's time to wind down. When all of these external cues happen in the hours leading to bed time, your body knows it is time to sleep.

Understanding how these cues work together is essential for knowing how good sleep works. Think of them like the pistons in a car engine. Internal and external cues for sleep, like the pistons, all need to work together in order to move the car forward. If one piston isn’t working, not only is the car slowed down but the performance of the other pistons is compromised. If you struggle with sleep, it is likely because the timing, strength, regularity, and presence of one or more of these cues is off, and it’s affecting the other cues as well. Bringing internal and external sleep cues “in synch” with one another is a huge step toward better sleep. Throughout this module, we discuss how to make this change. Now that you understand some of the basic things that happen inside you in order to bring about sleep, you're ready to learn about the benefits of good sleep, which we will discuss in the following section.

Contributors: Tommy Rowan & Bryce Klingonsmith