How sleep is regulated? Basics And Stages of Sleep Cycle

Posted in Insomnia Causes Symptoms Treatment

Suffering from sleeplessness?? Read process of sleep and the basics of the sleep cycle. Read how sleep mechanism is regulated in human beings. Understand physiology of sleep and sleep phases.


Basics of Sleep Cycle

Letter to the patient suffering from insomnia

Date: 24-Jul-2016


Dear friend,

Sleep Cycles

We have discussed the reasons and symptoms of insomnia in previous articles. But to understand more about our disorder under consideration, I feel its necessary to understand the physiology of sleep process. First of all, let we define the sleep.

What is sleep?

Sleep is recurring phenomenon for several hours every night in which nervous system becomes inactive, eyes are closed, body muscles remain in relax state and consciousness gets suspended.

We spend around one-third of our life in sleep!!!

Stages of Sleep Cycle in humansWhat makes us fall asleep? And what is it that tells us to wake up??

Is it a brain???

Ok. But it seems to be dysfunctional when we sleep. How it informs itself to wake up??


Well, the answer is ‘Circadian Clock’.


Circadian Clock and Sleep Cycle


Sleep Cycle Stages Circadian RhythmsCircadian Clock is the biological timer fitted in our bodies by the Mother Nature.

Let we discuss how this circadian clock regulates sleep-wake phenomenon in our body.

Dear friend, I am describing sleep cycle comprehensively keeping in mind that you are non-health professional.

Let’s begin!!!

There are two processes. One drives us for sleep and the other for wakefulness. The wakefulness process counteracts the sleep process and keeps us awake and alert. We keep on performing our activities and as time passes this process weakens.

Wakefulness process

As wakefulness process loses its counteracting strength, the sleep process takes over. Gradually the need of sleep develops in our mind and body. At a certain time, the neurons controlling wakefulness shut down. These neurons are situated in hypothalamus area of our brain. As soon as they shut down we sleep at that moment!!


There are other things that to be paralleled to switch off the functioning of these neurons .These neurons will not shut down if signals from other areas are negative.

Any Guess…what are those signals??

Signals like the stomach full with food, signals indicating a balanced state of emotions, signals arising from comfortable sleeping posture or signals of well-being and no pain.

Let we term them… signals of survival!!

And what is the result if such signals are negative???

Dear friend, we remain awake and alert!! As a result, we lose our sleep or quality of sleep!!!

Sleep process

Now let’s discuss the other process… the sleep process. As the neurons helping for our wakefulness turn off, sleep cycle kick off. We pass through different sleep cycle stages. Scientifically these stages are divided as NREM and REM.

Sleep Cycle Phases Circadian ClockLet’s try to understand NREM and REM sleep cycle. NREM stands for non-rapid eye-movement and REM stands for rapid eye-movement.  And they are divided based on EEG (electroencephalographic) patterns of brain activity during our sleep.

The first NREM-REM cycle is around 70 to 100 minutes. It, later on, keeps on lengthening. The second cycles last around 90 to 120 minutes. Theses cycles keep on alternating during our sleep. Scientifically NREM cycle is divided into four stages.

  • Usually, our sleep begins with NREM Stage 1 lasting around 1 to 7 minutes. At this stage, sleep can be interrupted by noise or touch.
  • NREM Stage 2 lasts for 10 to 25 minutes. It is believed that Stage 2 is responsible for memory consolidation.
  • NREM Stage 3 lasts for few minutes and NREM stage 4 lasts around 20 to 40 minutes.

Dear friend, all above categorization is based on EEG patterns.

The REM stage follows NREM stage which usually lasts around 1 to 5 minutes. As it ends NREM stage kicks off again. Again we repeat sleep cycle stages.

Overall REM sleep constitutes 20 percent of sleep cycle duration. Overall our sleep process is nothing but cycling of NREM and REM, one after another.

Sleep and Dreams

Dear Friend, most of our dreaming happens at REM stage.

Dreaming!! Let’s imagine… in your dream, you are jumping from the mountain!!!

However when you woke up… you find yourself in the same bed.


When sleep cycle arrives at REM stage, muscles lose their tone. Our reflexes remain relaxed. Hence physically we can’t react to the scenario that we are dreaming of.

And that’s why we do not fall out of bed!!!

But why we dream???

Probably our brain prepares for future defense! Probably it may be the exercise of a brain to consolidate memories!! Or maybe our brain processes emotions during the dreaming period!!!

We still don’t know the exact reason. But there are few research studies conducted. During the research, the study participants did not allow to enter in REM stage while sleeping.

And guess the research study results!!

Believe or not… all those participants were suffered from anxiety, stress, and depression!!!

Sleep Cycle Clock

What is Sleep CycleLet’s come to the basic question of this topic. How our brain initiates or stops our sleep? Which clock it refers to??

Well…our brain refers to biological clock. Our brain works as per circadian rhythm. Any biological process that displays oscillation of about 24 hours is termed as circadian rhythm. And such rhythms are driven by a circadian or biological clock.

Certain biological processes function or operate on a 24-hour clock cycle. Such cycles are further influenced by light or dark. Interestingly they are observed in plants and animals also.

Sleep-wake cycle is one of the examples of circadian rhythm in our body. Regulation of hormones and regulation of body temperature are few other examples.

Where this circadian system is situated?? Which body part regulates our sleep cycle??

Well… the biological clock situated in our brain coordinates all body clocks in perfect synchronization. The suprachiasmatic nucleus in our brain hypothalamus controls circadian rhythms. It contains around 20000 nerve cells. And our sleep cycle is circadian rhythm.

How this clock regulates sleep cycle??

Sleep Cycle Regulation

Like the mechanical watch, clock genes are components of this circadian clock. They interact with each other and make the auto-regulatory mechanism, in which its activation and repression cycle take about 24 hours. This is the reason why we observe the 24-hour pattern in a behavior of certain bodily processes.

Its blessing from the Mother Nature that we can override this cycle if its necessitated by environmental changes.

The suprachiasmatic nucleus is responsible for coordinating our sleep cycle parallel to day and night cycle. It also coordinates cycles of feeding, locomotor activities and hormones like corticosteroids. We can adjust sleep timing if needed. And this helps us to accommodate in stressful situations or changing circumstances.

In addition, there is one more function suprachiasmatic nucleus does!!

The suprachiasmatic nucleus also controls the pathway of melatonin hormone produced by the pineal gland. And Melatonin usually secreted at night which consolidates the circadian rhythms.

Got it??

In conclusion, our sleep cycle is controlled by suprachiasmatic nucleus. It also considers bodily inputs like signals of survival. And if the need arises we can override the mechanism and remain awake.

I know its hard to understand the physiology of sleep and sleep cycle stages especially if you are a non-health professional person.

But still, I found its utter necessary to post you these facts!!


Just to ask you few questions!!!

Consider you are habituated to sleep at a certain time. Let’s assume, 11:00 pm is your regular sleep time. Then can we say that your sleep cycle initiation time is 11:00 pm?? Or can we say your biological clock is tuned for 11:00 pm to start sleep cycle???

NREM Sleep CycleIf your answer is ‘Yes’ then I have another question for you.

Would you ignore sleep alarm of your biological clock and invest that vital time in doing unnecessary things??

Maybe in watching movies… or may be in enjoying parties???

If ‘yes’…then disorder of insomnia likely to knock the doors of your dream home.

I hope you would have understood the functioning of a sleep cycle. This letter will help you to respect our biological clock.

Always remember…. ignoring alarm of our sleep cycle may impact our survival!!!!


Dr. Suraj Awadhani

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