Have you ever wondered what happens when after we close our eyes and drift off to sleep?
While our bodies may appear to be inactive during this period, there’s a lot going on behind the scenes.
Sleep is not one homogeneous entity: It’s actually comprised of 5 distinct stages that repeat cyclically throughout the night.
Understanding how sleep stages work can help you work towards getting the most out of your sleep; and can make planning the most refreshing nap a breeze.
The Sleep Cycle
Your brain cycles through 5 different sleep stages throughout the night, with a full cycle taking 90-120 minutes to complete.
The first cycle progresses from Stage 1 all the way down to REM; then we cycle back out of deep sleep going from REM to Stage 1, then back down again. 
The duration and frequency of the different stages also changes over time. The REM component of each sleep cycle increases as the night goes on, so that REM effectively dominates the second half of the evening. Meanwhile, slow-wave (Stages 3 and 4) sleep decreases as morning approaches.
Additionally, we undergo 3 full cycles of non-REM sleep before finally settling into our first phase of REM approximately 1-3 hours after we first drift off. 
Each stage is unique and plays a specific role in the sleep process.
This is the transition period being waking and sleep. During this brief stage you actually drift in-and-out of sleep, and can be easily aroused by external stimuli. You may maintain awareness of your external surroundings, yet feel a sense of distance from them.
The brain starts to shift from the unsynchronized waves associated with waking, to slower, more predictable Alpha waves and finally into Theta waves.
The body enters a state of relaxation: Breathing becomes more regular, and your heart rate begins to slow down.