Sleep Stages and Cycles
"Sleep is the intermediate state between wakefulness and death; wakefulness being regarded as the active state of all the animal and intellectual functions, and death as that of their total suspension." - Dr. Robert MacNish, 1834
For many years, doctors and scientists believed that the brain was very inactive during sleep. This conclusion made sense. After all, since the human body appeared completely inactive during sleep, why would the brain be any different?
A Window to the Brain
In the 1920s, this assumption changed due to the invention of the electroencephalogram. Abbreviated EEG, this device measures brainwave activity from the surface of the head via little wires that have been pasted to the scalp.
Suddenly, we had a new window into the inner workings of the mind. The EEG determined that while awake, the brain was obviously quite active. To everyone's surprise, however, sleep was much more complex than anyone had anticipated.
Scientists observed that the electric activity goes through a variety of distinct stages. These stages would repeat themselves in a predictable fashion throughout the night.
What Do Sleep Cycles and Stages Mean for You?
Our sleep is essentially divided into four stages -- light sleep, medium sleep, deep sleep, and dream sleep (or REM sleep). The adult brain goes through a full sleep cycle every 90 minutes. During a typical sleep cycle, your brain starts off in light sleep, slowly goes into deeper stages of sleep, and then comes back out into lighter stages, after which it will often go into REM sleep.
During REM sleep, the brain is very active, and when observed with the EEG, can be just as active as the awake brain. Many think of dreams as occurring during a deep stage of sleep, but it's actually the opposite. Knowing that, it makes sense that we often wake up right after an especially vivid dream.
During the first part of the night, we spend a larger percentage of each sleep cycle in deep sleep. This is why it is rare to wake up during the first few hours of sleep.
As the night progresses and morning gets closer, we spent more time in light sleep and REM sleep. This is why dreaming is more common and more memorable towards the morning hours. The increase in lighter sleep also causes more awakenings closer to morning.
Waking up is not an abnormal part of sleep. In fact, it's actually protective. Small awakenings allow us to shift positions, and this movement is helpful to ensure that circulation is not interrupted to any particular body part for the entire night. If you've ever experienced your arm “falling asleep" because you slept on in for a few hours, imagine how it would feel if you slept on it the entire night.
Knowing about sleep stages and sleep cycles may explain how you feel and behave while your body is asleep. The complexity of sleep has certainly earned more respect than it had in the 1800s. And thanks to ongoing research, we are learning more every day even as we still have a long way to go to fully understand the complexity of sleep.