25 May 2020 Author : Elsa Parreira We still do not know the reason why we sleep. We know, however, the mechanisms that produce sleep . Sleep does not come from just one specific area of the brain; it results from the action of a neurons (nerve cells) spread throughout our brain. In this network there are: Activating neurons, which activate the cerebral cortex in order to stay awake; Neurons that have exactly the opposite function, that of falling asleep. Essentially, sleep is ‘on’ and ‘off’ because of interactions in brain neural networks. There are two fundamental mechanisms that cause us to sleep: Our biological rhythm; The so-called ‘sleep pressure’. The biological clock of the hypothalamus In our brain we have ‘a biological clock’ in a structure called the hypothalamus that tells us when we should be awake and when we should be sleeping . This natural clock is aligned with the light / dark cycle (day / night) of our planet and controls all circadian rhythms (which are repeated every 24 hours) of our organism. We must sleep at night when it is dark and we must remain awake during the day, when there is light. The hypothalamus regulates the production of melatonin , a key hormone for sleep, that depends on the presence or absence of sunlight. Melatonin production is maximal in the early evening to promote sleep onset. When we are exposed to light, melatonin production decreases. The hypothalamus sends signals to other brain structures, such as the brain stem, and this activates other areas such as the cerebral cortex, leading to the release of neurotransmitters that keep us awake or that induce us to sleep. Sleep pressure It is not just the existence of a clock inside our brain that causes us to fall asleep. We also sleep through the action of a homeostatic (organic balance) mechanism called 'sleep pressure', which reflects our brain need to sleep . It can be compared to the desire to eat when you are hungry. When we wake up in the morning, this pressure is almost zero, but as the day progresses, the sleep pressure gradually increases, reaching its maximum value at night, when we need to sleep. As soon as we fall asleep, the sleep pressure returns to very low again. If we sleep a nap longer than 30 minutes in the middle of the day the sleep pressure will drop dramatically at that time and then there is no time to return to the higher values that lead us to fall asleep at night. When we sleep little, the possibility of the occurrence of microsleeps increases. In the microsleeps we fall asleep involuntarily, for a very brief period (less than 3 seconds) without being aware or able to control. Microsleeps can have disastrous consequences, such as causing traffic or industrial accidents. Sleep phases and cycles Sleep is not a uniform and equal process throughout the night. It develops in several phases that are organized in cycles. The sleep cycle is repeated 4 to 6 times in one night . During the different phases of sleep, our brain emits different electrical signals that can be captured on an electroencephalogram. By analyzing the electrical activity of our brain and other physiological parameters, we can determine which phase of sleep the brain is in. The different stages of sleep have different function s. Each sleep cycle is divided into: Non-REM sleep; REM sleep. REM stands for rapid eye movements. Non-REM sleep Non-REM sleep, which lasts the longest and occurs first at night, consists of several phases: Phases of superficial sleep (transition from wakefulness to sleep); Deep sleep phase (with slow waves in the electroencephalogram) which is very important for its restful and restorative action. REM sleep REM sleep alternates with non-REM sleep. During REM sleep our eyes move under the eyelids, but the rest of the body is paralyzed while we dream intensely, our breathing speeds up and the electrical brain activity is very similar to what we experience when we are awake. The different phases of sleep intervene differently in consolidating memory. For example: During slow wave non-REM sleep (deep slow sleep) declarative memories (the memory of facts) and episodic memories are consolidated; During REM sleep, memories with emotional content are processed. During sleep, new connections are established between neurons and unnecessary ones are eliminated, restructuring the neuron networks. So a good night sleep improves our brain in functional terms . Meet the neurology team from Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
