Thursday, September 5, 2013
The mystery of sleep
Have you ever found yourself wondering why we sleep? Which hidden mechanism is the responsible of that apparently easy but indispensable function in our brain? Let us take a closer look of what happen while we are sleeping.
There is a transition between the wakefulness state and sleep state where the subject experiences a feeling of sleepiness and it exists a gradual disconnection from the environment around us. It is quite important the role of thalamus in this process since it acts like a "door" that blocks the stimuli coming from the outside. There are three main factors that lead us to lay in a comfortable place, close our eyes and take a little nap: vigilance factor, circadian factor and homeostatic factor.
Before doing a through analysis, we must consider the four nervous systems present in our brain that regulates both wakefulness and sleep states: reticular activating system (RAS), hypothalamic sleep system, REM sleep sign generator and suprachiasmatic circadian clock. The reticular activating system (RAS) is the responsible of wakefulness state, depending on whether or not it is working, we will be awake or sleeping, respectively. The hypothalamic sleep system does the opposite process. It will be working during sleep hours and will stop working while we are awake. The circadian clock controls the two previous systems by activating the first one and inhibiting the second during daylight hours, and making the opposite procedure during the night. The REM sleep sign generator has the role of regulating REM and Non-REM sleep cycles, which we will discuss further.
Once we have talked about these systems, we can explain the three most important factors in the sleep. The vigilance factor activates the reticular activating system (RAS) by determining the wakefulness state even when we are really tired or in the dead of night. All anxiety and emergency reactions can be included in this factor, and also those substances such as caffeine and drugs that act by keeping active the reticular system. The circadian factor ensures that we respect the sleeping hours at nigh. This function is carried out by the circadian clock, which is located in the hypothalamic suprachiasmatic area. The homeostatic factor maintain a proper balance by increasing the need to sleep in the case we have been awake longer than usual.
Thus, if we find ourselves in a dark place, whether or not at night, without any kind of problem or thought, and after a hard day of work, the previous three factors will activate the hypothalamic sleep system and inhibit the reticular activatig system (RAS). This way, the threshold of external stimuli will be higher and we will fall asleep.
Let's talk about REM sleep. Does everybody know what I am talking about? I think so.
It is named REM sleep due to the fast movements of our eyes during this cycle. In fact, REM means Rapid Eye Movement. REM and Non-REM phases are alternated during the sleep state. Non-REM phase is made up of three stages: the fist one is a transition from wakefulness to sleep state where eye movements are slowed down and muscle tone is reduced. When a person is woken up during this stage, he/she is convinced of not having fallen asleep. It becomes difficult to wake up during the second stage, and even more difficult during the third stage since we enter in a deeper sleep where there is a lack of oniric activity or eye movements. The EEG pattern shows slow brainwaves, and blood pressure along with heart rate and cardiac output decrease substantially. After this last stage of Non-REM phase, we enter into REM phase, which is also called paradoxical sleep since the EEG pattern is similar to wakefulness state, although the sleep is as deep as in the third stage. In REM phase, our body experiences eye movements, muscle spasms, atony in the muscles that maintain an upright posture, and, at last, vivid dreams. So, we dream during REM phase.
Both REM and Non-REM phases are alternated while we are sleeping. A new REM phase occurs every 90-100 minutes, so during an eight-hour sleep period we will experience more than five REM phases and their respective dreams. However, we usually remember the last dream before waking up, which it is part of the last REM phase.
The sleep pattern varies considerably over the years and there are some modifications in REM and Non-REM phases length.
We already explained how we fall asleep and the different phases of sleep, but why do we sleep? Unfortunately, nobody has found a definitive answer so far. It is thought that it could be related to the memory consolidation and brain restoration, but why does sleep have such a determining role in every species? It is still a mystery.
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I've read something a little different than what you've written here: Stage 1: You are entering into light sleep. This stage is characterized by Non-rapid eye movements (NREM), muscle relaxation, lowered body temperature and slowed heart rate. The body is preparing to enter into deep sleep. Stage 2: Also characterized by NREM, this stage is characterized by a further drop in body temperature and relaxation of the muscles. The body's immune system goes to work on repairing the day's damage. The endocrine glands secrete growth hormones, while blood is sent to the muscles to be reconditioned. In this stage, you are completely asleep. Stage 3: Still in the NREM stage, this is an even deeper sleep. Your metabolic levels are extremely slow. Stage 4: In this stage of sleep, your eyes move back and forth erratically as if watching something from underneath your eyelids. Referred to as REM sleep or delta sleep, this stage occurs at about 90-100 minutes after the onset of sleep. Your blood pressure rises, heart rate speeds up, respiration becomes erratic and brain activity increases. Your involuntary muscles also become paralyzed or immobilized. This stage is the most restorative part of sleep. Your mind is being revitalized and emotions is being fine tuned. The majority of your dreaming occurs in this stage. If you are awakened during this stage of sleep, you are more likely to remember your dreams. These stages repeat themselves throughout the night as you sleep. As the cycle repeats, you will spend less time in stages 1 to 3 and more time dreaming in stage 4. In other words, it will be quicker and quicker for you to get to stage 4 each time the cycle repeats.]
ReplyDeleteFirst of all thank you for your comment. The American Academy of Sleep Medicine (AASM)recently divided Non-REM phase into 3 stages: N1, N2, N3. Apart from this, what you wrote is something more than what I wrote and nothing different.
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