It might not look like it from the outside, but sleeping is actually quite a complicated business.
Your muscles might be relaxing, but your brain is quite busy while you sleep. And it’s running on a precise schedule.
Every night, you work your way through a number of sleep cycles lasting on average about 90-120 minutes. Each cycle is divided into four distinct stages.
Your sleep cycle starts with three stages of Non-REM (NREM) sleep and is concluded by one stage of REM sleep.
REM is an abbreviation for Rapid Eye Movement. During this phase of sleep, you dream the most deeply, as the eyes move quickly from side to side and brain activity increases.
Most of us have probably heard of REM sleep at one point or another, but the other three stages of sleep don’t get as much attention as they deserve.
Let’s take a look at what goes into getting through a sleep cycle.
Falling Asleep: The First Stage Of Light Sleep
Any restive night in bed begins with the process of first falling asleep. The very first stage of sleep is extremely subtle. So subtle that when awoken from the first stage of sleep, people will generally insist they never fell asleep.
The first stage of sleep is characterized by a feeling of drowsiness and slow eye movements. Muscles throughout the body begin to relax as brain wave activity gradually slows.
If you’ve ever experienced the sensation of falling or moving while falling asleep, that’s a good indicator of being in the first stage of sleep.
This is a very light stage of sleep and most people can be easily awakened during the first stage of sleep.
Relaxing Into Sleep’s Second Stage
While stage one is mostly a bridge between sleeping and waking, stage two is the first real stage of sleep. Most people spend between 40-60% of their time asleep in stage two.
In stage two, the body continues to relax. Brain waves slow further while heart rate and body temperature falls.
As the body relaxes, the brain performs important functions such as memory consolidation and performing essential tasks related to learning.
It is much harder to wake a person in stage two of sleep than it is in stage one.
Restoring The Body With Stage Three
After stage two is complete, the body enters into stage three of sleep.
This is the most restorative stage of sleep, sometimes referred to as “deep sleep.”.
During stage three, human growth hormone is released to help the body restore itself. Stage three has the strongest impact on reducing your sleep drive,
While brain waves fall to their lowest level of activity during stage three, it is believed that the brain is performing a sort of “maintenance” which allows it to refresh itself for learning and memory in the coming day.
People are hardest to wake when they are in stage three of sleep.
Activities known as “parasomnias” often occur in stage three. These include behaviors such as sleep walking, sleep talking, bed wetting, and night terrors.
Dreaming Of Tomorrow In Stage 4: REM Sleep
After the body has worked through the three stages of non-REM sleep, the REM stage can begin. To reach REM sleep, the body must go through all three prior stages -- it does not move from stage one or stage two into REM sleep.
The REM stage is associated with the most vivid dreams. While a person can dream throughout the sleep cycle, dreams are most vivid and pronounced during REM sleep.
On average, a person spends about 90 to 120 minutes in REM sleep each night. A person will typically experience between three and five REM stages in a night.
Each sleep cycle ends with a stage of REM sleep. After the REM stage is completed, the person returns to stage one and begins the cycle once again.
As you sleep, your REM stages will gradually become longer. While your initial REM stages will be comparatively short, as you continue to sleep your cycles will add more time in the REM stage, reducing the time spent in stage three.
This means that you experience your longest REM stage right before waking.
During REM sleep, breathing becomes more rapid and irregular as blood pressure rises. The eyes dart back and forth quickly, while the rest of the body’s muscles are paralyzed.
It is easier to wake a person during REM sleep than during stage three sleep. However, interrupting REM sleep often leaves a person feeling extremely groggy and tired.
While the exact function of REM sleep is not well understood, it appears that REM sleep plays an essential role for the brain's functioning and overall health. REM sleep may contribute to memory consolidation, learning, planning, imagination, and a number of other vital functions.
Cycles Within Cycles
Your sleep cycle is nested within an even bigger cycle known as your circadian rhythm.
A circadian rhythm regulates all sorts of essential behaviors, including your sleep and metabolism. It’s your circadian rhythm which lets your body know when it should begin feeling tired and heading to bed and when it is time to wake up.
Every human body comes equipped with a biological clock. Your body’s clock uses environmental cues such as sunlight and temperature to synchronize itself, but it also continues to work in the absence of any cues, working to synchronize with a roughly 24 hour day.
It’s this biological clock which dictates your circadian rhythm. Disrupting your sleep can have serious impacts on your circadian rhythm, which can have cascading effects which impact your entire body.
Improving Your Sleep By Understanding The Stages Of Sleep
Now that you know a little bit about the stages in your sleep cycle, you can use this information to help yourself get a better night’s sleep.
As we outlined above, while all stages of sleep are necessary, the later stages of sleep are the most restive and restorative. It is believed that stage three is the most restorative stage of sleep and the one responsible for the most repair to the body and the largest reduction in sleep drive.
REM sleep also performs essential functions for the mind. Being woken during REM sleep can cause heavy drowsiness.
With that in mind, it’s easy to conclude that in order to get a good night’s sleep, you need to stay asleep long enough for your body to complete multiple sleep cycles. For most people, this is around three to five cycles.
In other words, getting too little sleep or sleep interrupted before it reaches stages three and four prevents your body from performing some of sleep’s most essential functions. Without completing three to five sleep cycles a night, your body is not properly rested or recovered.
The ideal time to wake up is just as your body completes its longest REM stage of the night, after at least seven to eight hours of sleep.