We don’t realize it, but our body is very active while we sleep. Breathing, temperature, brain and heart activity… What happens in our body when we sleep, dream or when we are about to wake up?

Each year, we spend an average of 118 days sleeping. We don’t necessarily realize it, but our body is not inert and remains active even when sleeping. Throughout the night, our temperature, breathing, brain, heart and muscle activity, and hormone production change with each sleep phase. Discover what happens in our bodies when we sleep.

While falling asleep

Breathing becomes slower, and muscles relax. During this half-sleep phase, one may feel as if one is “falling” into a void or jerking.

The internal temperature of the body drops. We fall asleep at thermal defervescence, meaning that the body’s core temperature will drop at night to reach its lowest point between 4 and 6 am. Falling asleep in a room at about 19°C promotes sleep.

Melatonin – a hormone fundamental to falling asleep that regulates sleep/wake cycles – is secreted when it gets dark. Its production signals the brain that it’s time to go to sleep. “It is essential to know that melatonin secretion stops when the light level increases (in other words, when the sky is blue).

Therefore, blue light is particularly effective in stopping the secretion of melatonin and delaying sleep,” warns the neurologist. That’s why you should avoid looking at screens (television, smartphone, tablet …) before going to bed. The ideal is to set up a digital curfew one hour before sleeping.

Phases of sleep: paradoxical, slow, deep, duration of a cycle

Rest consists of successive stages: falling asleep, light slow-wave sleep, deep slow-wave sleep and REM sleep. How long does a sleep cycle last? What is the pattern of a normal sleep cycle? When do we dream the most? When is sleep the most restorative?

During light slow-wave sleep

Brain electrical activity slows down. Usually, when we are in a state of calm wakefulness, the brain has an electrical activity of 8 to 12 hertz. The brain has prolonged electrical activity, down to 1 hertz during deep, slow-wave sleep when you fall asleep.

Muscles become increasingly relaxed, and breathing becomes increasingly quiet. Nevertheless, during this phase of light slow-wave sleep, sleep is shallow and fragile: the slightest external stimulus (a noise or a change in brightness…) can bring us out of our sleep.

Cortisol levels, a hormone that helps the body cope with physical or psychological stress, among other things, decrease, which promotes sleep.

During deep, slow-wave sleep

• Breathing is prolonged and regular.
• The heart rate slows down, and the muscles are completely relaxed. And as the heart rate decreases, blood pressure also decreases and is at its lowest level.
• Brain activity is slowed down, but for all that, it is during this phase that the connection between neurons is most optimal, which helps to promote memorization and creativity, for example.  
• The pituitary gland secretes growth hormone in a pulsatile manner during this sleep phase. It is a hormone that promotes growth, reproduction, and cell repair. It also plays a significant role in different metabolic mechanisms (regulation of glucose in the blood, amino acids…), which is why it is said that deep slow-wave sleep is the most refreshing.

During REM sleep

The body is entirely inert and paralyzed. On the other hand, brain activity is at its peak, and under our closed eyelids, the eyes make rapid movements in all directions. This is why this phase is also called “rapid eye movement sleep”.

It is in this phase that we dream the most. “During REM sleep, the individual has electrical activity in the brain as if they were awake or just asleep. However, from a behavioural point of view, the individual is sleeping profoundly, and his muscles are paralyzed.

This is why this phase is called REM sleep,” explains the sleep specialist. The person simultaneously shows signs of deep sleep and signs of wakefulness.

Breathing is “more anarchic and dependent on his dreams. If the dream is very rich in emotions, the breathing rhythm is faster, for example,” our interviewer says. The pulse rate also increases.

The heart rate is more irregular and also depends on the dreams one has.

During waking up

The body temperature gradually increases, which will stimulate wakefulness. That’s why it’s recommended in the morning when you wake up to take a rather hot shower so as not to stress the muscles and to do some stretching to warm up the body and bring its temperature up faster, our interlocutor advises.

Brain and muscle activity intensifies. Micro-awakenings punctuate the “real” awakening we are often unaware of.

Cortisol is secreted between 6, and 8 am. Cortisol is stimulating and promotes waking up. Melatonin levels, on the other hand, are at their lowest,