Have You Known the Sleep Cycle That Happens during your Sleep?

What Happens When We are Sleeping
What Happens When We Sleep?

Do you know when we sleep there is a cycle that occurs repeatedly in our brain exactly at the first time feeling sleepy until awakened? In which, each cycle contains some different stages that continuously alternate going through our night’s sleep until awaken.

The stages are broadly grouped as NREM and REM. NREM represents stage 1, stage 2, stage 3, and stage 4. However, REM has no division.

NREM is Non-Rapid Eye Movement that indicates no eyes motion if tracked by the EOG. Along with the second division, REM is Rapid Eye Movement which shows significant eyes motion as the effect of dreaming.

So, when we are trying to fall asleep until dreaming, the sequence should be entering stage 1 firstly, then stage 2, stage 3, stage 4, go back to stage 3, stage 2, and then dreaming. In short, NREM is the first and REM is the second. That is also called as the first cycle of sleep.

If converted to a list, the steps during the first cycle sorted like this:

  1. Feels sleepy (Stage 1)
  2. Fall asleep, light sleep (Stage 2)
  3. Deep sleep (Stage 3)
  4. Deeper sleep (Stage 4)
  5. Deep sleep (Stage 3)
  6. Light sleep (Stage 2)
  7. Dreaming (REM)

The second cycle is uniquely no stage 1 present. It directly goes through the stage 2, stage 3, stage 4, then get back to stage 3, stage 2, and then dreaming (REM).

The second cycle runs like the following list:

  1. Light sleep (Stage 2)
  2. Deep sleep (Stage 3)
  3. Deeper sleep (Stage 4)
  4. Deep sleep (Stage 3)
  5. Light sleep (Stage 2)
  6. Dreaming (REM)

Next with the third cycle that is similar to the preceding cycle but the stage 4 disappears and REM lengthens as the time goes on. By passing through to the stage 2, stage 3, then stage 2 and dreaming, this third cycle seems shorter than the preceding cycle.

Sleep Cycles

So what are stage 1, stage 2, stage 3, and stage 4? What happens in our body at those stages? How about the dreaming stage (REM)? You would know them by reading further the explanation below.

Stage 1

It is described as the condition which body feels drowsiness. We feel eyes so heavy and body really wants to relax. We, at this stage, are essentially still able to respond the outside stimuli such as sounds but tend to ignore it. Additionally, a person being at this stage is definitely still easy to get awakened.

If tracked by EEG (Electroencephalography) machine, the brain exhibits its activity by showing beta, alpha, and theta waves. Beta wave is considered as wakefulness state as well as body performs at its fully responsive condition. The accompanying wave, alpha, is considered as relaxation state such as meditation where eyes closed yet still conscious. Apart from beta and alpha, the most important wave at stage 1 is the present of theta waves which show the state of slowing brain activity that leads to falling asleep, in other words, drowsiness.

Stage 1 (drowsiness) occurs one time and only at the first cycle. Body temperature and respiratory rate become normal. That may be the reason why cold place often makes us sleepy because it affects our body temperature down and leads to sleep.

Stage 2

At this stage, we enter the first true stage of sleep or best described as falling asleep. Although we still hear sounds, however, we don’t understand what about we heard of. If any conversation heard, we don’t know really what they talk about, sounds like the whispering wind or quiet conversation.

If tracked by EEG, the waves show sleep spindles and K-complex waves. The sleep spindle is a burst fast high-frequency wave which occurs for half a second. The counterpart, K-complex is a uniquely short peak negative waves followed by peak positive wave which goes on for 1-2 minutes. Compared to the previous stage, Beta and alpha waves start to disappear and leave theta waves dominating stage 2.

Sleep spindles and K-complex

The picture below is a recorded brain-wave by the EEG which shows the waves of someone entering stage 2. You could see the present of sleep spindles and K-complex signed by the red rectangle below.

Tracked Stage 2 Sleep by EEG

Stage 3

Those who being at this stage 3 are getting into the deep sleep and extremely difficult to get awakened. If awakened, badly groggy may happen. Our ability to respond any stimuli really disappears, the body takes place at unaware state.

Theta waves have been lost and brainwaves represent delta waves more frequently up to 50% of the total waves. Interspersed with sleep spindles and K-complex which still sometimes appear yet less frequent rather than at stage 2.

Stage 3 is commonly referred as deep sleep or slow wave sleep (SWS). This is the vital moment of the brain for memory consolidation.

Stage 4

Going further to the next stage, stage 4, which is actually similar to stage 3 but what distinguishes them is the present of delta waves that dominate the total waves (more than 50% of the waves) This stage only occurs during the first two cycles of the sleep for a normal people. Meanwhile, during the latter cycle stage finds out stage 4 lost and only stage 3.

As the same as like stage 3, it’s totally hard to wake sleepers at this stage anyway. We have total inability to respond stimuli.

Stage REM

Dreaming is the most interesting experience during REM stage. If awakened for a quite long time, we can remember it vividly in the morning afterward. Meanwhile, if it’s just a moment, it may fade away, surely we can’t remember it clearly.

During dreaming, our brain is active like waking state but muscle is strangely paralyzed. As brain becomes more active, it affects to eye movement which turns on and very easy to indicate. Brainwaves at REM stage is dominated by alpha waves like meditation. The following tracked brainwaves exhibit us the person who is at stage REM.

Besides the eye movement, the other body’s measurements such as heart rate, blood pressure, and respiratory rate show the increasing level of activity. It marks that body turns active like waking state.

Final Comprehension

Keep in mind that sleep is not about time to body turns off. As our brain is one of our most active body’s part during sleep, it regulates sleep cycles we never aware of it.

Along with the substitution of each stage, in our body, there occur some changes such as heart rate, body temperature, muscle tone, blood pressure, eye movement, etc.

 Stage 1Stage 2Stage 3Stage 4REM
DescriptionDrowsiness, easy to wake upFall asleep, Light sleep, still easy to wake upDeep sleep, hard to wake upDeep sleep, hard to wake upDreaming
Waves- Alpha
- Beta
- Theta
- Theta
- K-complex
- Sleep spindle
- Theta
- Delta
- Delta- Alpha
- Beta
Eye MovementSlowSlowNoNoYes
Muscle ToneDeclineLowLowNoParalyzed
Heart RateSlight DeclineSlight DeclineDeclineDeclineIncreasing
Blood PressureSlight DeclineSlight DeclineDeclineDeclineIncreasing
Respiratory RateNormalSlight DeclineDeclineDeclineIncreasing
Body TemperatureNormalSlight DeclineDeclineDecline

Those above are probably some pieces of fact behind the other unrevealed evidence about what happens to our body when we are sleeping. Recognizing them really helps us to both manage and get a better sleep.

Now you may have understood why you feel yourself dreaming last night and why not. Now you surely realize you are a light sleeper because your stage 2 takes longer in your sleep cycle. Now you need more stage 3 and 4 to get a full restorative sleep. Now you have a plan to hack your body by creating a cool bedroom to fall asleep faster. I am sure, the knowledge served on this page definitely benefits you in recognizing yourself deeper and understand how to manage it.



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