Sleeping seems pretty straightforward. After all, we’ve all done it since we were babies. Animals do it, too, without even thinking about it. The need to sleep seems to be hardwired into us and it’s something we usually do automatically.

However, defining sleep is a complex task. What with all the neurotransmitters involved, the physiological changes that take place when we sleep, and the different brain wave patterns that occur when we sleep, it can be hard to figure out what, exactly, sleep is.

Physical Changes in Sleep

The most obvious way to define sleep is based on what we see, or what can be most easily monitored. These include some of the more obvious changes we see when a person falls asleep.

For instance, people who are sleeping have much lower activity levels than those who are awake. Even vivid dreamers or sleepwalkers aren’t as active as they are when they’re wide awake. Sleeping people are also less responsive to stimuli coming from outside themselves. Sure, loud noises, earthquakes, and bright lights will eventually wake a sleeper, but it can take a while. Most sleepers will be oblivious to soft noise, dim light, and light movement.

Sleeping people also tend to be lying down. This is true across cultures and throughout time. Even animals lie down to sleep.

All of these things can indicate to us that a person is asleep, and so have some place in our definition of sleep. A person is asleep when they have these characteristics.

Neurotransmitters in Sleep

Neurotransmitters are brain chemicals. They help neurons communicate and signal to the brain that it is time to do certain things. The neurotransmitters present in the brain and their relative concentration changes when we’re asleep. Another way to define sleep, then, is based on which neurotransmitters are present and active.

Adenosine is a neurotransmitter that is closely tied to sleep. Scientists aren’t sure, yet, if it causes sleep or is a byproduct of something else that does the job. What they do know is that adenosine builds up in the body when we are awake. At some point, it seems to be involved in the process of indicating to the brain that it’s time to sleep. Then, as we sleep, it gets broken down.

By the time we wake, if we’re getting enough rest, there is very little adenosine in the body and the process starts over again. Measuring levels of adenosine, then, can be one way to define whether a person is awake or asleep.

Other neurotransmitters are also involved in sleep. Melatonin, for instance, seems to indicate to the body that it’s time to go to sleep, and possibly facilitates some the changes that take place in the brain as we fall asleep. It is also broken down while we rest.

Serotonin, which is necessary for the production of melatonin, also changes in concentration with sleeping and waking, as does norepinephrine, another neurotransmitter. Together, these seem to facilitate sleep by their presence or absence.
Brain Waves and Sleep

Brain Waves and Sleep

The most common scientific definition of sleep among scientists has to do with the firing pattern of neurons. When we’re awake, our neurons are relatively active and they seem to fire randomly. When we fall asleep, this all changes.

In non-REM sleep, our neurons fire much slower than when we’re awake, and they do so in coordinated patterns. In REM sleep, they are much more active — sometimes even more active than when we’re awake! This firing is much more like waking firing in that it seems random. When we’re done with an REM cycle, the neurons go back to slow, syncopated patterns. Researchers can monitor these cycles and use their presence or absence to decide whether or not someone is asleep.

Sleep may be harder to define than you’d thought, but it’s not impossible. Our functional definitions of sleep, based on what we can see, seem to usually follow the scientific definitions, based on neurotransmitter concentration and brain wave patterns. As researchers learn more, we will hopefully develop more consistent definitions…and we may even learn how to sleep better!