Each night the body goes through several different phases: Light Sleep, Deep Sleep, REM sleep, and Wakefulness.
Over the course of an eight-hour night of sleep, these sleep phases repeat within 4 to 5 cycles between bedtime and morning. The transitions between each of the sleep phases are gradual and not always easy to remember or detect.
But before we get ahead of ourselves, let’s start with the basics first.
The phases of sleep
During the night, the mind and body go through many changes as they move through a very predictable and regular pattern of sleep phases. The different phases of sleep are Light sleep, Deep sleep and REM sleep. These sleep phases are often interrupted during the night by wakefulness, periods of disruption to sleep. Light sleep accounts for about half of the night and bridges the gap between the other sleep phases.
Deep and REM sleep are the two (very different) components of restorative sleep, the periods during the night when the body undergoes the its most significant mental and physical restoration.
Differences between Deep and REM
These two phases look very different from each other in terms of brain waves. While Deep sleep has big slow waves due to the synchronous brain activity characteristic of that stage, REM sleep has activity that’s highly variable. At times, our brains in REM sleep are so active that sleep technicians can sometimes confuse it with Wake – that’s how similar the signals can look.
What is a sleep cycle?
A sleep cycle is a period during the night in which you go through each of the sleep phases, and perhaps some wakefulness. A typical sleep cycle involves: going into light sleep, which deepens and can become deep sleep, especially earlier in the night, then back into light sleep which then transitions into REM sleep.
It is very common to wake up either as you enter or exit REM sleep while transitioning out of or into light sleep. This is the most natural time to wake up and can occur during any of the usual 4-5 sleep cycles over the course of a night.
Notice that the body does not simply go into a sleep that just gets deeper and deeper over the course of the night until you wake. Sleep deepens and lightens several times over the course of a night, and waking up is often a normal part of this natural process.
Moving from phase to phase
The transitions between the different sleep phases are usually very fluid and can occur over the course of several minutes.
This is also the case when transitioning into and out of wakefulness. It is very difficult to pinpoint the exact point at which you fell asleep or woke up in large part because falling asleep and waking up do not happen with the flip of a switch.
For instance, it is common to wake up out of light sleep without even realising you were asleep, especially if you are doing something other than trying to fall asleep, such as watching television or driving. (Drowsy driving is very dangerous and should be avoided).
Not able to remember?
It is also possible to forget that you have awakened during the night.
Have you ever had the experience of turning off your alarm in the morning, going back to sleep, and then having no recollection of having awakened to turn off your alarm? This is because short-term memory turns off as you drift into one of the sleep phases. This sleep-related short-term memory loss is common, and nothing to worry about.