Sleep is an extraordinary piece of machinery that researchers are just starting to understand.
If neuroscientists begin to reveal the secrets of our nights, they are still far from having unraveled all of dreams’ mysteries. Because the mechanics that punctuate our nocturnal rest are really complex.
Our sleep consists of several cycles of ninety minutes on average. If we do not go to sleep at the first signs of fatigue (yawning, heavy eyelids), we miss the first cycle and will have to wait for the next.
Like a traveler who stays on the platform until the next train. Each cycle is divided into several stages (light, deep, REM sleep) which have specific functions.
Slow light sleep: the most fragile
Ah! This delicious moment when we fall asleep.!
For a few minutes already, our thoughts have been appeased. Throughout the day, our brain activity gradually released molecules such as serotonin and adenosine that cause sleep. After a while, their accumulation makes us fall into the arms of Morpheus.
At the same time, the decrease in brightness signals to our internal clock, located in the hypothalamus, that it is time to go to bed. This signal lowers our internal temperature and triggers the secretion of melatonin. This hormone which will be produced throughout the night makes us want to sleep in an irrepressible way.
But beware! At this point, the slightest external stimulus – a slamming door or a ray of light – can still wake us up easily. If nothing disturbs us, our brain activity progressively decreases and our muscles relax. Breathing becomes calmer.
In a few minutes, we reach stage 1: it’s official, we sleep. Ten minutes later, we slip into stage 2, during which sleep is less light even if it remains fragile. The electrical waves emitted by the brain are a little slower still. The level of cortisol – the stress hormone – decreases. Heart rate and blood pressure drop. This phase accounts for about 50% of all night’s sleep.
Slow deep sleep: crucial for memory
When we reach stage 3, we are in deep sleep.
So much so that a person awakened during this phase remains completely dazed during the first seconds. This period lasts between ten and twenty minutes per cycle. Heart pressure and muscle activity are at their lowest level.
This is when we rest the most.
Our brain is also idling and yet … This is a key time for memorization. The connections between neurons multiply in order to fix in memory knowledge and memories.
It is also during this phase that the secretion of growth hormone reaches its climax. Deep sleep accounts for 20-25% of total sleep time, and is mainly concentrated during the first part of the night.
Paradoxical sleep: the time of dreams
Under our closed eyelids, our eyes move in all directions, while the rest of our body is inert, as if paralyzed. This is why it is called REM, which stands for Rapid Eye Movements.
Conversely, the respiratory rate and the pulse increase.
An electroencephalogram would reveal that our brain activity is at its peak. As intensely, if not more, than during awakening. And yet … we are sleeping!
According to scientists, memorization of motor skills (learning to ride a bike for example) would take place mainly during this stage. Some studies also show that it stimulates creativity.
So if you have a dream of wild horses full of amazing adventures, this could be happening during REM sleep.
If dreams can occur throughout the cycle, it is during REM sleep that they are the longest and most complex, without anyone really knowing why.
Shorter at the start of the night, this phase gradually lengthens over cycles and occupies between 20 and 25% of our sleep time.
Sweet wake up
REM sleep is followed by micro-awakenings of which we are rarely aware.
Scientists think that these, in the middle of the night, may help us to ensure that “everything is fine” by increasing our vigilance. At the end of the last cycle, when the day begins to break, the light acts like a bugle in our brain even when we have our eyes closed, and warns our biological clock that it is time to press the switch ” awakening ”.
After a good night’s sleep, the concentration of adenosine and melatonin, the molecules of sleep, is at its lowest. While that of cortisol goes up. These two signals promote our awakening, now imminent. It’s off for a new day.