Everyone has already experienced these two scenarios. Sometimes, we wake up with the impression of having lived an extraordinary adventure during the night, more or less strange and more or less unpleasant, in such places, with such people, etc. A scenario so eccentric that we often wonder what it can reveal about our unconscious... But sometimes, we don't remember anything at all, as if we hadn't dreamed for a single second. Yet everyone dreams. Each night. So, what can condition these reminiscences?
To begin, remember that sleep is broken down into several phases:falling asleep, light slow sleep, deep slow sleep and paradoxical sleep. During the night, these phases are repeated cyclically:there are on average 3 to 6 cycles of about 60 to 120 minutes each. It is during REM sleep that we dream. Our brain activity is then very intense and our eyes perform incessant rapid movements (we also speak of "REM sleep", for rapid eye movement ).
REM sleep typically accounts for 20-25% of total sleep time, and lasts longer in later sleep cycles (which partly explains why we sometimes wake up in the morning feeling like we're awake in the middle of a night). 'a dream). Note that dreams can also occur during light slow sleep, but they are much less intense and correspond more to abstract ideas than to real dream representations.
Many studies have shown that sleep helps the brain store new memories. Others have raised the possibility that sleep is also an opportunity for the brain to eliminate excess information. Specialists agree that as soon as we come out of REM sleep to enter slow-wave sleep, it only takes about ten minutes for the dreams to disappear from our memory.
If we do not always remember our dreams, it is a priori because our neurons are in charge of “sorting” through all the information that we accumulate. This is at least the conclusion of a study published in September 2019 in Science :from experiments on mouse models, the researchers discovered that neurons producing the melano-concentrating hormone (denoted MCH), located in the hypothalamus, actively contribute to the forgetting of dreams.
These neurons, which are activated especially during REM sleep, would indeed be able to differentiate memories that are important for our psyche from those of lesser interest. The former are then stored in our unconscious, while the others are simply erased from our memory – in this case, the MCH neurons send inhibitory messages to the hippocampus, which is the memory center of the brain. And they would do the same with dream sequences:in other words, if our dreams are deemed uninteresting, they are discarded.
Don't worry, if you never remember your dreams, that doesn't necessarily mean they're pointless! Simply, it means that the memories gathered during the day, during the waking phase, are more important - and the space being limited, they deserve to be stored in memory more than your dreams.
To sum up, remembering a dream in detail involves two conditions:it must occur towards the end of sleep time — when the REM phase is longer — and it must be interesting enough to be maintained. in memory.
Another study published in 2017 in the journal Frontiers in Human Neuroscience , also reported that external auditory stimuli could also play a role in the memorization of dreams; these noises would cause very brief awakenings, long enough to allow the individual concerned to remember their dreams. Thus, people who are generally particularly sensitive to auditory stimuli - whether during wakefulness or sleep - would be more prone to micro-awakenings and would remember their dreams much better.
In 2018, the same team of researchers reported that the frequency of dream recall is also associated with the white matter density of the medial prefrontal cortex, which is found to be higher in people who recall their dreams; it is moreover the first study which highlights cerebral structural differences between “big dreamers” and “little dreamers”. That said, the researchers concluded that the medial prefrontal cortex plays more of a role in producing dreams than in remembering them. The lack of significant difference between the two groups at the level of the amygdala and the hippocampus further supported the idea that these two regions are not directly involved in the frequency of dream memory.
In conclusion, we are not all physiologically equal before the wonderful world of dreams, but one thing is certain:we dream even when we do not remember it. This is the conclusion of a 2015 study of people suffering from what is called REM sleep behavior disorder (RTSP) - which is expressed by abnormal nocturnal behaviors (screams, sudden and violent movements, etc.), because unlike the “healthy” sleeper, the sleeper suffering from PTSD can move during REM sleep. These externalizations of the dream, clearly visible to observers, suggest that people who never remember their dreams are still able to produce them. “The production of dreams is universal, the ability to remember them is variable concluded the researchers.
