Each night the body goes through several different phases: Light sleep, Deep sleep, REM sleep, and Wakefulness.
These sleep phases go through 3-6 cycles each night. The transitions between each of the 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.
Deep and REM sleep are the two (very different) components of Restorative sleep, the times during the night when the body undergoes the most mental and physical restoration. They also look very differently from a brain wave standpoint.
While Deep sleep has big slow waves due to the syncronous brain activity charateristic of that stage, REM sleep has activity that's all over the place. At times, our brains in REM sleep are so active that sleep technicians (and Zeo) can sometimes confuse it with Wake - that's how similar the signals can look.
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 3-6 sleep cycles over the course of a night.
Notice that the body does not simply go into a sleep which just gets deeper and deeper over the course of the night until you wake up. Sleep deepens and lightens several times over the course of a night; waking up is often a normal part of this natural process.
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 realizing you were asleep, especially if you are doing something other than trying to fall asleep - such as watching TV or driving (drowsy driving is very dangerous and should be avoided).
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 woken up to turn off your alarm? This is because short-term memory turns off as you drift into one of the sleep phases.
It's also quite common, so don't worry too much if this has happened.