Every time you fall asleep, you pass through four different sleep stages that, when put together, form a sleep cycle. What you may not know is that during one core sleep period, a person can pass through multiple sleep cycles. But that’s not all that’s fascinating about what happens when we sleep.
Explaining Sleep Cycle Stages
There are two different kinds of sleep stages: non-REM and REM sleep. The non-REM stages are divided into three individual phases. Each time you shift from one stage to another, something different happens with your body and your mind.
— Stage 1
During stage 1, you fall into a very light state of drowsiness. You are easy to wake up when you’re in this stage, and if you do, you might not even be aware of the fact that you had fallen asleep. Awaking from this stage is usually sudden, because your body starts to relax, but not to the fullest.
In this stage, you slowly shift from being awake to the next stage of sleep. A lot of people experience hypnic jerks, which are basically involuntary twitches and muscle spasms. Other people have reported a sensation similar to falling, as they start to lose control of their body when they doze off.
— Stage 2
A lot of experts consider this to be the first true stage of sleeping. You are less likely to wake up suddenly compared to the first stage. Your brain waves start to change as you are now sleeping, and they slow down considerably. People who like to take power naps should wake up after this stage.
During stage 2, your brain activity now becomes a mix of K complexes and sleep spindles, which have the role of protecting the brain from sudden awakening. Your heart starts to slow down as it relaxes, and your body temperature decreases. If you’ve ever woken up without being covered by a blanket in the middle of the night, it’s because of this body temperature drop that can be uncomfortable for some.
— Stage 3
The third sleeping stage goes by many different names, like deep sleep or delta sleep. This is the point where your sleep is most restorative, as your brain waves have a very slow speed. When you’re in the deep sleep stage, you are unlikely to wake up due to mild external stimuli (you will still wake up if you hear loud noises, for example).
We mentioned before that people who take power naps should wake up after the second stage of sleep, and there’s perfectly valid reasoning behind that. The third stage of sleep reduces a person’s sleep drive, which basically means that your body needs a limited amount of deep sleep within a 24-hour period.
If you take really long naps during the day and allow your body and mind to reach the third sleeping phase, you will find it much harder to fall asleep at night because you don’t need as much rest as someone who didn’t sleep at all during that same day. However, if you take a short 1-hour nap in the afternoon and you don’t reach this third phase while sleeping, you will find it perfectly natural to fall asleep at a reasonable hour that very same night.
Deep sleep is considered to be the most restorative sleep phase because that’s when your growth hormones are released. Your body and your muscles will regenerate faster, preparing for the upcoming day. It is also when your immune system works to fight off any bacteria or viruses that may be present in your body.
It is difficult to wake up when you’re in the delta sleep stage. Furthermore, someone who is suffering from sleep deprivation may spend more time than usual in stage 3. If someone suffers from parasomnias (such as night terrors or sleepwalking), this is the stage when they normally occur.
Despite knowing all of this information, researchers believe they still know very little about the deep sleep stage. Some speculate that it’s because of this deep sleep stage that the brain is prepared for a new day, basically refreshing itself.
— Stage 4
This is the REM sleep stage, and probably the most intensely-studied one of them all. This is when dreaming occurs, so it’s why people have taken a lot of interesting in analyzing a person’s brain waves during this stage.
It is considered the stage of vivid dreaming, when rapid eye movement is registered because of the mixed frequency of the brain waves. In fact, studies have shown that the brain waves of a person who is in the REM sleep stage are strikingly similar to the one they have when they’re awake.
This information surfaced in 1953, when people were able to use machines in order to monitor a person’s brain activity. Before that, it was a common belief that people had zero brain activity while sleeping.
There is plenty of controversy about REM sleep and how it affects people on an emotional level. It is known that during REM sleep, the part of the brain known as the amygdala is active. The amygdala is in charge of our emotions, which could explain why people are sometimes affected by what they dream.
While studies have shown that turning short-term to long-term knowledge occurs in the non-REM sleep stages, it is believed that in the REM stage, people play out these memories with a fantastic twist while they’re dreaming.
The Full Sleep Cycle
On average, a complete sleep cycle can take between 90 to 120 minutes. As you fall asleep, you progress through the four stages of sleep we’ve mentioned earlier. Naturally, since you sleep more than 120 minutes every night, you will pass through the sleep cycles over and over again, until you wake up.
What’s interesting is that people don’t, however, move from stage 3 to stage 4 without any intermission. As you sleep, you pass from the lightest sleep stage to your deep sleep (that’s stage 1 to stage 3), but instead of moving to REM sleep (which is stage 4), you go back to stage 1 and then move to the REM phase. Basically, a sleep cycle is something like: stage 1 – stage 2, stage 3 – stage 2 – stage 1 – REM.
When you are ready to fall asleep, you slowly drift towards stage 1, which is typically a very short sleeping stage. Then, you transition to the second stage, which is generally longer than the first. For most people, stage 6 accounts for 40 to 60 percent of their sleep time.
Next, you move to stage 3, the restorative stage of sleep. Stage 3 is shorted compared to stage 2 and makes up for about five to 15 percent of your sleep cycle. This deep sleep stage usually lasts longer for children and teenagers.
The REM sleep stage can kick in at any time during the sleep cycle, but studies have suggested that it usually makes an entrance about 90 minutes after sleep onset. When the first REM stage is complete, your sleep cycle will continue with the other three stages. The first sleep cycle takes about 90 minutes, and the following ones are typically longer and can reach even 120 minutes.
About Light Sleep
If stage 3 is considered the stage of deep sleep, stages 1 and 2 are light sleep stages. Stage 1 accounts for about three percent of the total sleep cycle, and adults typically spend most of their time sleeping in the second stage.
During light sleep, people are more likely to move, but may sometimes experience dreams. It is also very likely that if you wake up and recall what you dreamt in this stage, it won’t make much sense to you. This could be because researchers suggest that in the light sleeping stage, you might be putting together image fragments of what you experienced throughout the day. Your dreams won’t have a narrative, as this is more likely to occur in REM sleep.
Even if light sleep is not as restorative as deep sleep, nor as dream-worthy as REM sleep, it does play a very important role in your sleep cycle. When you’re in the light sleep stages, your cells do repair, even if not at a pace as fast as that of the deep sleep stage.
Neuroscientists believe that this is the time when your daily-accumulated knowledge is transferred into memories, which basically means that this is when you transfer new knowledge into long-term information, thus helping you learn new information.
You might have heard about light sleepers and heavy sleepers before. These are, in fact, terms used to describe the stages that individuals most sleep in. For example, light sleepers are more likely to wake up due to external stimuli because they spend most of their time in the first two sleeping stages. Heavy sleepers spend more time in deep sleep and are more difficult to wake up.
You may have heard about circadian rhythms by now, but there is another important sleeping mechanism that could explain more about sleep cycles: homeostasis.
Circadian rhythms are our natural internal clocks that operate in the back of our brain the entire time. Because of them, the human brain cycles between sleepiness and alertness throughout the entire day. They are responsible for regulating our sleep/wake cycles.
But circadian rhythms do so much more than that. They direct our metabolism and even our hormone release. This internal biological clock is synchronized with several internal and external cues, such as the fact that natural daylight dictates the hours when people are more likely to wake up.
Homeostasis is what tracks a person’s need for sleep. Because of it, the body knows when it feels tired and is time to rest. Thanks to homeostasis, you feel sleepier with each hour you spend away, and as this state of tiredness builds up, you are more likely to experience deep sleep once you go to bed.
A Word on Parasomnias
Parasomnias deserve a section of their own because they are sleep disorders that include out of the ordinary sleep-related behavior
- Sleepwalking is probably the most well-known form of parasomnia. It involves getting out of bed and walking despite the fact that you are very much asleep. This occurs during the non-REM stages of your sleep, with no memory of these actions once you wake up.
- Sleep terrors are something that people confuse with nightmares quite often. People who experience them will wake up suddenly feeling terrified, with an increased heart and breathing rate. They normally occur during non-REM sleep and are typically short in duration (although there have been cases of night terrors lasting up to 40 minutes).
- Nightmares are more like vivid dreams that occur during REM sleep. When a nightmare occurs, the person wakes up instantly (most of the time, but that’s not a rule) and is almost in a state of shock. Some people are unable to tell what they were dreaming about, but plenty of them have trouble getting back to sleep.
- Sleep paralysis can be quite scary, because it occurs when you’re awake, but are unable to move your limbs or body. Sleep paralysis occurs when you’re either falling asleep or waking up, and such an episode typically lasts a few seconds.
- Confusional arousals tend to appear when someone is woken up from their deep sleep stage. If you’ve ever woken up in the middle of the night and just sat on the bed confused and mostly unaware of your surroundings, then you too have experienced confusional arousal. The more people try to interact with you, the longer it takes to shake off this confusing state.
During a person’s core sleeping hours, they pass through a number of different sleep cycles, as their length varies between 90 to 120 minutes.
There is not very much you can do to influence your sleep cycle without drastically changing your sleeping patterns. A sleep cycle consists of four different stages, each with its own characteristics and important part to play in the overall efficiency of your resting hours.