The Human Sleep Cycle
The phenomenon we call sleep is something virtually everyone is well versed with. However, the main question that springs to mind is; how does the sleep process actually work? What actually happens when you drift off to sleep, there’s actually more to it than your body simply switching off at night and then rebooting in the morning like an electronic gadget.
Over the years, researchers have tried to understand the alternating cycles that our sleep patterns follow. Initially, sleep was just assumed to be a period of passive rest, but today with further research and modern equipment we are able to get a better understanding of what is really happening when you enter the land of nod.
Have you ever wondered why sometimes you wake up feeling refreshed? While other times you wake up feeling tired and groggy? Well this is generally because of the impact you’re your sleep has had on your body. What scientists are now understanding is that sleep can be a very complex process that the human body undergoes and it is responsible for the rejuvenation of both the body and the mind.
Hopefully after reading this piece you will have a better understanding of how sleep works and a general overview of its impact and importance in our lives. From the different stages of sleep to the reason for dreams hopefully you will learn something new about sleep and maybe even about yourself.
This article aims to discuss the various stages of the human sleep cycle. This is to give you a better understanding of the whole process so you can have a better understanding as to how to improve your sleep and learn what may be impacting your ability to sleep well.
So the first obvious question is how can anybody i.e. scientist’s measure sleep with any degree of accuracy? Well to test the different stages of sleep, scientists use a Polysomnography. This is a machine that works by measuring different body functions simultaneously. For instance, brainwave activity, eye movement, heart rhythm, respiration, and muscle activity to name but a few. These individual measurements when combined and analysed in a particular way give sleep experts a system to calculate the quality and quantity of sleep of the person connected to the machine.
The funny thing is that every human on this earth needs sleep in order to function, in the same way we need to drink water in order to function and survive, yet many people never actually ask the question why do we need sleep in the first place? Or what does the sleep process actually involve?
Well let’s start off by outlining the different sleep stages which scientists have discovered each of our bodies go through when we sleep!
Human Sleep Stages
Basically, our bodies go through 5 stages when we are asleep. Four of the stages are known as Non-REM Stages (Non-Rapid Eye Movement) while the Fifth stage is known as REM stage (Rapid Eye Movement Stage). In some cases, there is a sixth stage. This stage features relaxation and wakefulness which is the stage where someone falls asleep.
Over the course of a night time, the body goes through the sleep cycle four to six times. In each stage, the body will spend on average at least 90 minutes in each one. Each of these stages has a unique and restorative purpose to the human body. This includes; hormonal regulation as well as consolidation of the memory among other functions.
For that reason, it is essential to let the body cycle through all the sleep stages. When you lack a good night’s sleep, both your mind and body are deprived of the essential elements that help you maneuver through the following day’s activities are impacted upon in a detrimental way.
The Sleep Stages
There are five sleep stages. The first four stages are the Non-REM stages while the other last one is REM.
Stage 1; this stage is characterized by a drift in and out of sleep, and it is at this point that you can be awakened very easily with the slightest noise or sensory interference. In the stage one of Non-REM, there is a slower movement of the eyes as well as a slowdown in the muscle activity. At the first stage of sleep, most people experience contractions of muscles which is then followed by a strange sensation of falling which may be stronger in some people.
Stage 2; in this stage, you experience slower brain waves and the movement of the eyes begins to slow and stop. However, you can experience a rapid burst of brain waves. Here your body prepares for a deeper sleep. The temperature of the body drops significantly and the heart rate also slows down.
Stage 3; when you enter the third stage of sleep, you are likely to experience faster brain waves which are put in place of the slower brain waves known as the delta waves. With this stage you now begin to slip into a deeper sleep. It is worth noting that it is at this stage that some people may experience things like sleepwalking, talking during the night, night terrors as well as bedwetting. These kinds of habits are referred to as parasomnias. They majorly occur during the Non-Rem to REM sleep transition.
Stage 4; in the fourth stage of the sleep cycle, the brain happens to produce some delta waves exclusively. If you wake someone at this stage, they tend to feel disoriented during the first few minutes.
Stage 5; now this is the REM stage also known as the Rapid Eye Movement stage during sleep. At this stage, the eyes tend to mimic the activity of waking up. Although the eyes may remain closed, they tend to move rapidly from one side to the other. This is induced by the intense brain activity and dreams that occur at this stage.
What is the Human Sleep Cycle?
The Human sleep cycle is the period when an individual takes progresses through all the designated stages of sleep outlined above. As you have seen from the different stages, an individual does not move from deep sleep straight to REM sleep. There is a set cycle that is followed.
Instead, the sleep cycle moves progressively. From the Non-REM stages to light sleep then the deep sleep. Later from the deep sleep to the light sleep then REM sleep and so on. Below is a more practical order of the stages and how they progress.
- Stage 1– The Light sleep
- Stage 2– The Light sleep
- Stage 3– The Deep sleep
- Stage 2-The Light sleep
- Stage 1– The Light sleep
- The REM Sleep
Right after you pass the REM sleep, you tend to return to the first stage of light sleep and then begin a whole new cycle. As you progress through the night, you spend more time at the REM sleep with lesser time in the Deep sleep.
The first cycle of sleep takes precisely 90 minutes. Right after that, the cycle may average in between 100 and 120 minutes. In a night, your body can undertake at least four and five of these sleep cycles.
The Deep Sleep
The stages 3 and 4 represent the deep sleep, delta sleep or the slow wave sleep. It’s very hard to wake up someone from this stage of sleep, especially younger children. That is why they are likely to wet their beds during this stage of sleep. In this stage of sleep, it is unlikely to see any muscle activity or even eye movements.
Among all sleep stages, deep sleep can either reduce someone’s sleep drive or provide an individual with significantly more restorative sleep. That is why you are still able to fall asleep during the night even if you had a quick short nap during the day.
However, if you have a long enough nap and drift into deep sleep during the daytime, it’s likely that you will have a hard time getting to sleep at night. This is because there will be lesser need for the body to fall asleep.
During the deep sleep period, there is the release of a specific hormone responsible for human growth which strives to restore your muscles as well as the entire body after the long day of work and exertion it has undergone. It is at this stage also that your immune system gets the chance to restore itself fully.
It has also been suggested that during the deep sleep stage of sleep the brain refreshes itself to accommodate learning experiences of the next day.
When does the REM Sleep Stage Occur?
The slow wave kind of sleep develops in the early stages, most notably during the first half of the night and then REM occurs in the later half. REM kicks in usually around the 90 minute mark right after you have fallen asleep.
The first cycle of REM lasts precisely 10 minutes while each successful REM cycle after that lasts a little bit longer. The first stage of REM lasts for a 1-hour period or thereabouts. The average person experiences about three or five cycles of REM in a single night.
After the REM period, you are likely to wake up. And, if the waking up period happens to lasts for a longer period, you are likely to remember everything the following morning. When the wake period is more abrupt we tend to forget any dreams and memory of the night’s sleep is a bit cloudy.
The REM period is generally categorized by heavy, shallow and irregular breathing. The eyes may jerk in rapid movements and the limb muscles may be paralyzed temporarily. Brain waves in the REM stage are as intense as the ones that occur when someone is awake.
As well as that, there is an increase in the heart rate as well as a rise in the blood pressure. In some instances, the body may lose its ability to keep the body temperature in check so temperature fluctuations can be prevalent.
Brain Waves during the Sleep Cycle
There are two to three ECG channels which are recorded to determine whether the patient is awake or indeed to determine which sleeping stage they are in. With ECG, four types of brain waves are captured. These waves occur either when someone is awake or sleeping. The waves are measured in cycles per second.
These kinds of waves occur when someone is awake. They have the lowest amplitude and the highest frequency when compared to all the other waves. These waves show a huge variability.
These waves occur when a person is awake or when they are relaxing. For instance, during meditation. These waves tend to be slower with a lesser amplitude as well as having more variability than the beta waves.
The theta waves occur during the first and second stages of sleep. They are slower in their frequency but the amplitude is greater than in alpha waves.
As one progresses from stage one to stage two the activity of theta waves continues also and there is the sudden increase in the wave frequency and amplitude.
During the third stage of sleep the waves are very short with a very high amplitude. This is because delta sleep happens to be the deepest sleep of all.
At Which Stage Do Dreams Occur?
At the REM stage of the human sleep cycle, this is when the most vivid dreams happen. This is because at this stage there is more brain activity. If you are woken up at the REM stage, you are likely to remember these dreams, many can appear so real that you can feel like the dreamed events actually occurred although once you regain your senses your brain determines this not to be the case.
In REM sleep there can be many cases of muscle paralysis too. Scientists who have studied this have said that this is usually a way that the body tries to protect us from any injuries when our bodies begin to act up during sleep.
In a single night, you may dream from four to six times. Whether or not you remember your dream, it still occurs.
REM sleep was discovered after there was a development of machines that could monitor the brain activity. However, before the introduction of these machines, the scientists believed that brain activity did not happen during sleep.
Scientists have also recently discovered the theory that the deprivation of the REM sleep can have such an effect that it can lead to insanity. In that line of study, they have realized that lack of REM sleep can increase the chances of clinical depression also. REM is also linked in a major way to memory function and learning.
How does my body know when to fall asleep?
Well, this is usually a question that might drive you crazy too. Do you fall asleep just because it’s dark? Or is it a routine? The answer lies in the circadian rhythm. This rhythm which is a regular body rhythm runs within a 24-hour period. The Circadian rhythm is also referred to as the biological clock.
Daylight is one of the primary inducers of circadian rhythms. That is why changes in time zones usually affect your sleeping.
The circadian rhythm is responsible for controlling your drowsiness at night and your extra energy in the morning. However, the rhythm can change depending on your sleeping habits. For most adults, the biggest drop in energy usually happens at night and after lunchtime.
The times will change for those morning people and late night owls. You will not feel the effects of the circadian rhythms if you are asleep.
If we have no control over our Circadian Rhythm, then how come we end up sleeping at night?
The answer is very simple. When there is darkness, your body sends out direct signals to your hypothalamus. The result is that the hypothalamus then releases melatonin which makes your entire body feel tired and sleepy.
That is the main reason why your daytime and night time tends to coincide directly with your circadian rhythms. That being said, you need to keep an eye on your sleeping and waking up routines in order to achieve a balance of both.
When it comes to the sleep cycles throughout the night, they can last for different periods of time depending on the circumstance of each sleeper. The Non-REM stage tends to dominate the better part of the night while the REM stage increases progressively during the second half. However, the time spent in each of these stages changes with time and according to one’s age. Did you know that your age plays a major part in the amount of sleep you should be getting every night?
Click here to find out exactly how much sleep you should get every night?
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