Sleep is a necessary part of life, at least if we talk about animal life: we have not been able to find any animal more complex than a fly that does not sleep: Apparently shortly after reaching the milestone of having a central nervous system, evolution begins to needing that system to go into an “offline mode” (Zadra & Stickgold, 2021).
Unlike what happened until the end of the last century, today we know what our body does at night. While we sleep, we carry out a series of processes necessary for life; such as the cleaning of dangerous excesses of beta-amyloid in the brain (Ju., 2017) or the release of hormones (Van Cauter, 2011; Mong, et al., 2011). We even know that sleep is necessary to consolidate motor (Walker et al., 2002) and semantic learning (Djonlagic et al. 2009). However, all this only tells us that evolution decided that maintenance be done at night, but not because… we can undoubtedly think of a brain that, while awake, secretes growth hormones or insulin or some mechanism for cleaning the brain. excess amyloid that does not involve being in airplane mode. This is far from obvious: sleeping is extremely dangerous for a living being in its wild context, since we are more vulnerable to predators and not only us but, at night, our fellow humans who could be “on guard” as well. They suffer from drowsiness, so we are easy prey. This led Allan Rechtschaffen to say in 1979 that “if sleep does not serve any vital function, it is the biggest mistake that evolution has made” (Cited by Zadra & Stickgold, 2021).
What is so important that we can only do offline? Perhaps our dreams are a clue, since it seems to be one of the few things we can only do without being awake, or with the help of hallucinogens.
Dreams are cognitions dependent on the non-wakeful state, with hallucinatory characteristics (we see things that are not there and we temporarily believe in their existence) with certain distinctive characteristics: they are highly emotional and vivid, they are strongly visual, they have a particular narrative structure (discontinuous, with changes in theme and characters) a certain level of apparent symbolism, a privileged access to our memories and, at the same time, a tendency to subsequent amnesia, since we remember a tiny part of what we dream (McNamara, 2019).
Since there have been written records, human beings “interpret” dreams: generally as visits from gods or spirits, and in the 19th and 20th centuries, as involuntary messages from an unconscious that fought to be heard over a very quiet censorship, an idea that psychoanalysts possibly borrowed from 19th century German philosophy (Crews, 2017). Later, a contrary current advocated the epiphenomenological nature of dreams: when we sleep, visual areas of the brain are activated (especially in ) and our brain subsequently constructs a narrative to try to explain the presence of photographic and fragmentary memories: in In reality, the dream narrative is a fabrication of the newly awakened, because humans need everything to make sense (McCarley & Hobson, 1977).
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Dreams. Photo on Pexels.
Possibly both approaches are wrong: for messages, after analyzing the content of tens of thousands of dreams, science does not find that there is a very clear relationship with a particular theme (for example: sex, incest, etc.). It could be said that this is because the unconscious “disguises” the content… but let’s admit that current artificial intelligence is good enough at cryptography to outperform an outdated psychoanalyst and no: only people who are concerned about sex dream more about sex, and Those who are about to buy one dream more about waterproof jackets… and so on. On the other hand, it is unlikely that something as resource-intensive as dreaming will serve no purpose. When we dream, our brain consumes a lot of glucose, much more than would be necessary to simply be able to do what we think we do when we sleep in terms of “cleaning.” Why so much waste? Obviously we need to dream (Walker, 2019).
Just as the key to why we sleep possibly lies in why we dream, the key to the latter, as one of concepts, possibly lies in the least understood concept in psychology: consciousness. As with dreams, theories about their existence have ranged from their ethereal nature to their practical uselessness… But today we have a pretty clear picture. We know that consciousness is not necessary for many things our mind does: solving problems, storing memories, perceiving, motor and verbal learning, etc. But when we ask ourselves what it is that we cannot do if we are not aware; The unanimous answer seems to be this: associate multimodal concepts (from several perceptual channels) in unified representations of reality (Dehane, 2019; Graziano 2019). This is because what we call consciousness seems to be the massive and coordinated activation of a large part of our brain based on a specific set of stimuli (a problem, a sensation, ourselves).
While we often associate sleep with unconsciousness, this is not accurate: we are not aware of our external world and have lost an important capacity for critical judgment, but we are strongly aware of the hallucinatory perceptions that constitute our dreams: we live them. This is possibly due, precisely, to the fact that during the stages where our dreams are more elaborate (REM phase) the activation pattern of our brain is very similar to what we have when we are awake and letting ourselves be carried away by the flow of our ideas, as when we walk or passively look at the landscape. Every time we are not focused on something specific, our brain activates a specific and very broad network that scientists have called the default activation network (in English, Default Network Mode; Domhoff, 2018). In those daytime moments, we are aware of our thoughts, but with an almost spectator role… we are able to report that we are thinking if we are asked, but we may not remember it a while later, more or less what happens with dreams when we just woke up.
The explanation does not end here, apparently our way of thinking while sleeping is different and complementary to the one we have during the day. Some lexical decision experiments carried out by Stickgold have allowed us to understand that during our REM phase, our preference when associating concepts is different from the one we have while we are awake… while we prefer strong associations between concepts (dog-animal or dog -cat) when someone is abruptly awakened in REM and asked to associate one word with another, they tend to prefer words that are not directly related, but are not incoherent, but rather are weaker associations (“dog-security” or “dog-meat”). In these experiments, people rarely chose “dog-cat” but they also did not choose combinations like “dog-heater.” It was clear that, in those moments, our minds look for alternative but logical paths. Precisely as if a complex indexing of the information was being carried out that would allow its rapid recovery not only through direct paths… more or less what allows Google to deduce that when we put “vacation” in the search engine it can offer us an advertisement for sports clothing , because we are surely going to need it (Stickgold et al., 1999; Zandra & Stickgold, 2021).
We thus arrive at the end with an interesting provisional answer: we sleep because we need to dream, and we need to dream because living beings, apparently, need to have a state of activation similar to that of wakefulness, but without worrying about moving or perceiving external stimuli, so in order to process the information of the daytime period in a finished way. We need, in other words, to be conscious but asleep for a good amount of time each day to correctly process new information and correctly index it with old information.
Bibliographic references:
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