Last Updated: December 8, 2025
When the sleep pressure is sufficient, and the biological clock gives the signal, the signs of falling asleep are felt (yawning, discomfort in the neck, chills, etc.).
Knowing how to spot them has many advantages because they are the first train of sleep arrival. Now is the time to go to bed!
Their appearance is linked to the activation of predominant biological mechanisms during sleep:
- Cortisol (stress hormone) decreases
- The heart rate drops,
- Body temperature is reduced,
- Synthesis of melatonin (sleep hormone) begins.
At the same time, all phenomena related to awakening decrease: hunger, thirst, but also social and sensory factors.
During the day, when we are awake, brain waves (indicators of brain activity) have a high frequency, and the body is activated continuously to respond to any requests and situations that arise.
When the first phase of sleep appears, that of falling asleep, these brain waves gradually decrease. The perception of the environment is more and more confused; the body begins to relax. This state is close to that encountered when we take a micro-nap and only last a few tens of minutes.
Slow Sleep
It includes two main stages: light slow sleep, and deep, slow sleep. It is characterized by decreased brain activity and relaxed muscle tone.
During the light sleep stage, the phenomena that appear become more pronounced, and we lose contact with the outside world over the minutes. In the beginning, we have a waltz-hesitation between sleeping and not sleeping, between micro-sleeps and micro-arousals. At this stage, muscle tone begins to decrease, and we can also notice this instability by the small movements of the body that can make us jump.
When the light sleep stage sets in, the eyes move less and less, the heart rate and breathing become slower and slower. On the other hand, the electrical activity of the brain is irregular. This stage alone accounts for half of the total sleep period.
During the deep sleep stage, the brain waves are slow and complete (between 0.5 and 2 Hz), which means that the brain’s activity is minimal. On the physical level, breathing is full and calm, muscle tone is weaker, and eye movements are absent, blood pressure no longer varies.
It is during this phase of sleep that it becomes difficult to wake the sleeping person.
What also characterizes deep sleep is that it is present mainly at the start of the night during the first two sleep trains. It is the most physically recovering stadium.
REM sleep
The last car in the sleep train is the paradoxical sleep train, so named because it presents a paradox between a brain that resumes a level of activity very close to, or even higher than that of waking, and an inert body. This phase of sleep represents about 25% of the total time of our nights.
The storm of paradoxical sleep follows the calm of deep sleep.
At this point, if the body is entirely inert, the heart rate, blood pressure, and breathing, on the other hand, become irregular. The eyes also start to move quickly, hence the REM stage (rapid eye movements) to describe REM sleep.
Moreover, REM sleep is commonly referred to as dream sleep. At this stage of sleep, dreams can sometimes be remembered. While such reminiscence is impossible for dreams that occur during slow sleep.
If deep sleep is present mainly at the beginning of the night, on the other hand, the duration of REM sleep is more and more important as the night progresses.
Likewise, if the physical recovery takes place during deep sleep, everything related to the brain happens mainly during REM sleep:
- For example, procedural memorization (that of reasoning, know-how, learning gleaned during the day)
- Or the maintenance of psychological balance.
Then, at the end of this paradoxical sleep, we are more or less aware of an awakening phase. We can then switch to a new sleep train or, on the contrary, not be able to fall asleep again.
These nocturnal awakenings or difficulty maintaining sleep, which mainly occurs at the end of a sleep cycle, can have multiple causes. Still, natural solutions exist to help get the night back to normal.