Sleep seems pretty basic, right? You close your eyes, your brain turns off for several hours, sometimes you dream and sometimes these dreams make sense, then you wake up and continue on with your life.
Scientists have found, though, that sleep is anything but basic. In fact, some parts of the brain are more active during sleep than they are when you’re awake. There is more to sleep than meets the eye, and more than anyone even knows.
Characteristics of Sleep
One way to define sleep is to look at the different characteristics it holds. Some of these are common sense things that we all know because we have slept and we’ve seen others sleep. Some, though, are complex and fascinating and have come as the product of painstaking research.
Sleep is characterized by:
- Reduced activity. Even people who walk or talk in their sleep are less active than they are when they are awake.
- A change in posture. Most people, and in fact most creatures, sleep lying down. While there is some variation in how they lie (elevated or not, and on their backs, sides, or stomachs), people across time and across cultures have most commonly slept this way. Some people can sleep in other positions, especially when they’re exhausted, but lying down is the easiest and most common way to sleep.
- Being relatively easy to reverse. It’s pretty easy to wake up someone who is sleeping. Even if they are in a deep sleep and wake up groggy, they can be woken within just a few minutes. This makes sleep different than being in a coma or being knocked out. It even makes it different than hibernation.
- A drop in blood pressure. Unless the body isn’t functioning well, blood pressure is almost always lower when we are asleep. This can partially be accounted for by the fact that there are fewer demands on the body when we’re sleeping, but the drop is consistent enough that there may be other causes, too.
- Specific brain wave activity. Neurons in the brain seem to fire randomly while we are awake, and they are very active. When we sleep, though, our neurons slow down and begin to fire, when they fire, in specific and coordinated patterns. When we enter REM sleep, the neurons pick up their activity, sometimes becoming even more active than they are when we’re awake. As we exit REM sleep, the brain moves back into slow, regular patterns of firing.
While all of these are part of the current definition of sleep, the brain wave patterns are what scientists most often use to define the sleep state. When our neurons are firing in particular patterns, we are asleep. The rest of the time, we’re awake.
Neurotransmitters and Sleep
We can also define sleep by the set of neurotransmitters currently active in the brain. These brain chemicals change when we are asleep. Thus, sleeping or waking status can be determined by the set and balance of neurotransmitters currently active.
Scientists don’t understand exactly how this works, yet. What they do know is that these chemicals interact with neurons at the base of the brain, which act as a switch. They switch off the balance of neurotransmitters that keep us awake and begin to replace them with others, which help us sleep.
This may all tie back to a particular neurotransmitter called adenosine. This builds up in the body while we are awake. When it reaches a certain concentration, we begin to feel tired and feel the effects of not sleeping. As we sleep, this chemical breaks down until concentrations are low once again. Research has not shown definitively how this process works, but it does seem that part of the definition of sleep has to do with how much adenosine is currently in the bloodstream.
Researchers are always seeking to understand more about how our bodies work and the mechanisms by which we sleep. While the current answers to, “What is sleep?” leave something to be desired, science is pursuing some promising lines of research to add to our understanding and, hopefully, help us sleep better.