Humans have been theorizing about the answer to this question for most of our recorded history. While the jury is still out on why we need sleep, modern technological advances, especially the introduction of the electroencephalogram or EEG, made it possible for scientists to study sleep in new ways. This is why sleep science has seen more progress in the last century than at any other time in history. This technology allowed scientists and doctors to create a real picture of what happens when we are sleep for the first time and provided the information needed to define the multiple phases of sleep we experience each night. Understanding the structure of “normal” sleep gave sleep doctors and scientists a basis from which to identify sleep disorders and instances of disordered sleep and made the current field of sleep science possible.
That foundational understanding, which has evolved over time, is based around the idea that normal sleep is comprised of several different stages. Each stage is characterized by specific brain activity that can be seen on an EEG. While researchers are still uncovering the intricacies of what is going on in the brain, body, and mind at each stage, this basic understanding has made it possible to identify and treat many of the most common sleep disorders. Here is what we now know about what happens when we sleep.
Stage 1 – Light Sleep
As soon as you close your eyes, you begin transitioning from being awake to being asleep. This is the settling in period when your body settles in, your breathing slows, and you drift away from consciousness. Your brain waves shifts from beta waves to alpha waves and then settles in a pattern of theta waves. This is the stage of sleep where people report feeling as if they were falling and experiencing random, jerking limb movements. This stage only occurs once when you first fall asleep unlike other stages that are often repeated over the course of the night.
Stage 2 – Unconscious Sleep
During the second stage of sleep, your body temperature decreases and your heart rate slows down. Brain waves are slower but show bursts of rapid activity called sleep spindles. At this stage, you are ready to enter deep sleep and your brain and body begin to have different experiences.
Stage 3 and 4 – Deep Sleep
As you enter the third and fourth stage, you move into the deepest kind of sleep and your brain moves to the slow, deep pattern of the delta wave. It will be difficult to wake you up during these two stages of sleep which are the final NREM or non-rapid eye movement stages. Several sleep disorders, called parasomnias, occur during these stages including sleep walking, night terrors, and restless leg syndrome.
The last stage of sleep is generally referred to only as REM sleep and is characterized by the rapid eye movements that give this stage its name. Although your body remains still and quiet, your brain is almost as active during REM sleep as it is when you are awake. REM sleep is where you experience most of your dreams. During REM sleep, the normally shared experience of your body and brain fully diverges. This is primarily due to a type of voluntary muscle paralysis encountered during this stage called atonia which keeps your physical body from performing the actions occurring in your brain.
The Sleep Cycle
Although the sleep stages are labeled numerically, you don’t always experience them in order. Over the course of a single night, you may go through 4-5 sleep cycles. For most people, the first cycle of the night will follow the linear path from stage 1 to stage 4 before changing to a less linear pattern that looks more like this 1 2 3 4 3 2 REM – 2 3 4 3 2 REM -2 3 4 3 2 REM – 2 3 4 3 2 REM 2.
While there is still much to learn about why we sleep, what happens in our bodies and brains during sleep, and what functions sleep performs, this basic understanding of the structure of sleep created the foundation for modern sleep science.