The guardian once published an article (in 2013) called ‘the 20 biggest questions in science’.
Most of them are still considered to be the biggest questions in science and we want to pick out one of them:
Why do we dream?
Feel free to write whatever comes to your mind! You might see your response in the print version of issue 3!
I am not sure about the detailed science, but I remember reading REM sleep is when the brain consolidates memories and when we dream so maybe it has to do with replaying the days events.
We also know that performance declines without regular sleep (but maybe independent of dreaming?). Maybe the brain needs to reduce its processing load to clear the reactive oxygen species and other damaging chemicals produced during the day. Just my two cents.
One theory why we dream suggests that dreams help to maintain and train the fight-or-flight response as the amygdala is most active during dreaming. And way more active during sleep than when you are awake. Luckily, for us, this is counteracted by the brainstem that inhibits muscle contraction, leading to relaxation of the muscles during sleep. I like this theory, even though I don\’t believe that this is the reason why we dream.
As far as I know there are several different theories and models to answer this question. I’d like to mention an early one from Freud. Who suggested that dreams are (for) “…disguised fulfillments of repressed wishes.”. However we know that other animals (e.g. cats, dogs, some birds) dream as well and we currently have no way to clarify the connection between their dreams and wishes.
In the end, dreams are end-products (or side products) of some neurobiological processes. I think a better theory should also be able to answer “Why do we remember our dreams sometimes and sometimes not?”, “Why dreaming is preserved across species?”, “Why brain has an unrealistic image of the world during sleep?”.
Strange thing, that common rules do not apply in dreams most of the time. As a vivid dreamer, I cannot complain being in such a “Alice in Wonderland” like world time to time. And the question remains: why?
As someone working on memory consolidation during sleep I could imagine that dreams are an epiphenomenon of the reactivation of recently encoded autobiographical memories. We know that such replay occurs and that it is coordinated across the hippocampus and those neocortical brain regions coding for the multimodel facets of the experience . The few important moments of the day are repeatedly replayed so that the connections between those distributed co-activated neuronal populations are strengthened via Hebbian learning. Less important experiences on the other hand will be forgotten over time. These processes are usually associated with deep sleep – which allows memory networks to be updated and strengthened without too much interfering sensory input. Dreams often do not feel like accurate recapitulations of actual experiences. This might be a result of several autobiographical memories being replayed at the same time – while our prefrontal cortex is too sleepy to perform the usual plausibility checks .
However, there is another interesting model stating that during REM sleep the brain creates virtual realities in order to optimize the generative model it uses to predict future sensory input . The brain can thereby minimize surprise (the improbability of an input given the current model) and thus entropy (the average surprise over time), and as a result can defeat the second law of thermodynamics (yes, it gets wild here). To that end, the brain has to shut down its ability to evaluate prediction errors (the difference between model predictions and actually encountered sensory input). Observable signatures of this shut down during REM sleep are e.g. the loss of muscle tone (no efference copy – no movement) or the inability to regulate our body temperature (that’s why you sleep badly when it’s too cold or too warm). The upside is that we can dream without acting all of it out in the real world.
 The latter is pure speculation.
To just have a guess why we dream without scientific basis i just thought about the aspect, that during the night the input to the brain is stopped so:
During the day, the brain is confronted with a mass of new information, which must be analysed and sorted to existing stereotypes, pattern and memories. To “think” about details ore make the decision which information’s were important and must be shifted to the long-term memory is more the duty of the brain during the night – no extrinsic stimuli can interrupt also emotional aspects. For both – memory and emotions the limbic system is essential and very active while sleeping. Therefore, it could be important to have a time in which the brain reflects, consolidates, and do emotional validations with a much higher capacity without interruption from the outside and without being controlled by more strategical and high order brain regions.
Dreaming can be defined as subjective experiences during sleep. The astonishing fact is that while dreaming we experience the dream world including dream characters, settings etc. as real (with a few exceptions called lucid dreams). The capability of the brain is of course also active during waking, just with a few electromagnetic waves (light) and other sensory input to produce these subjective experiences. As most researchers now believe that we dream constantly while sleeping, i.e., the mind never sleeps, the question of a possible function or functions of dreaming arises. Although there have been a lot of theories over the years, starting from Sigmund Freud’s idea the dreams protect sleep, the major problem is that it seems impossible to test the functions of dreams directly. For example, Rosalind Cartwright did a study on divorced women, those who dreamed about their ex-husband were better adapted after one year with the idea that the dreams work through the stressful situation and, thus, well-being is restored. But, you can also argue that remembering the dream, recording it and thinking about it during waking might have been the basis for the positive effect of these dreams. I.e., you cannot differentiate between the effect between the dreamed dream and the dream processed in waking as you have to have a report in order to know what the dream is about. This might change if “dream reading” is a possible option. First attempts decoding sleep onset dreams were published by Horikawa and collegues.
Dream research produced a lot of data how dream content is related to previous waking-life experiences. Interestingly, emotional salient experiences are more likely to be integrated into dreams, and social interactions are very prominent in dreams compared to cogntive activities like reading, writing etc., even in students who spent a lot of time with these cognitive theories. So, one of the most recent theories is the Social Simulation Theory proposed by Antti Revonsuo. The idea is that the dream world provides an opportunity to train social skills which were of importance for our distant ancester because exclusion from the tribe might decrease the chances of reproduction (evolutionary approach). This seems plausible and a more general view was proposed by Allan Hobson.
As already mentioned in previous posts, one question is whether dreaming is reflecting processes of sleep-dependent memory consolidation. Unfortunately, research findings are scarce and inhomogeneous (see Schredl 2017). The basic assumption why the brain is able to consolidate waking-life experiences/tasks is that there is some form of replay during sleep. But whether this replay is limited on the system level or neuronal level or is reflected in the experiences of dreaming is still an open question. As mentioned above, these questions can only be resolved if imagaging of the sleeping brain is so sophistacated that we can match brain activation patterns with dream content.
Hobson, J. A. (2009). REM sleep and dreaming: towards a theory of protoconsciousness. Nature Reviews Neuroscience, 10(11), 803-813.
Horikawa, T., Tamaki, M., Miyawaki, Y., & Kamitani, Y. (2013). Neural decoding of visual imagery during sleep. Science, 340(6132), 639-642.
Revonsuo, A., Tuominen, J., & Valli, K. (2015). The Avatars in the Machine. In T. K. Metzinger & J. M. Windt (Eds.), Open MIND (pp. 1-28). Frankfurt am Main: MIND Group.
Schredl, M. (2017). Is dreaming related to sleep-dependent memory consolidation? In N. Axmacher & B. Rasch (Eds.), Cognitive neuroscience of memory consolidation (pp. 161-172). Cham, Switzerland: Springer.
I often dream of things that happened during the day or from the days before. Therefore, I think we dream so that the brain can process information/feelings/concerns in order to create space for new information. It is also interesting – at least for me – what we feel during our dreams. Dreams are not just a “movie” but rather contain emotions as well. Sometimes, I dream of things that I wish would come true, or people I would like to meet again, people who are not among us anymore. And it makes me feel like they were there with me again. Some people claim that if you dream about a certain thing, it has a meaning. For example, if you dream of blood it means that the person will experience a new start. I am not sure about that.
These are the two reasons why I think we dream: 1) to recharge the brain and 2) to give our emotions (which we often suppress) the space to come up.
Depending on the optimism of the observer, this question might either be seen as a delightful scientific puzzle or as an intellectual headache. As a scientist, it is my first instinct to reply to any ‘why’ question with a researched, detailed mechanistic account. However, as this angle has been well covered by other respondents, I will elaborate on what dreaming means to me from a personal perspective. But I do not for a moment suppose that dreaming has actually evolved for my personal entertainment.
Full disclosure, I love dreaming. I find it wonderful to be able to experience things that would otherwise remain unattainable and I am continually fascinated by the odd things that my mind comes up with and the otherworldly way in which dreams play with my memories, perception and conscious awareness. Thus, for me personally, the purpose of dreams is to entertain and provide food for thought.
The relationship between dreams and memories is rather curious. I often feel certainty that I’ve been dreaming without any memory of the content of the dream, just as if I had just run a marathon, feeling the exertion in every fibre of my body, yet having no conscious memory of the race itself. How can one know one was dreaming without remembering the dream?
Another interesting contradiction in dreaming is evidenced by the fact that one frequently feels like passive participant in a story, a character that things are happening to, yet the entire story is a concoction of one’s own mind. How do these two clearly distinct planes of conscious control emerge?
Furthermore, dreams prompt us to question what is real and inspire fantastical sci-fi works like The Matrix or Inception. Dreaming also opens up the discussion around varied states of consciousness since, even without experimenting with mind-altering drugs, everyone has some personal experience with dreaming.
Dreaming poses lots of intellectually stimulating questions while simultaneously giving me the ability to fly. I couldn’t ask for more.
We dream because only dreaming drives us to go beyond the status quo, beyond injustice of all kind ranging from social, political to gender inequality. If we do dream, a better world is possible. But if we instead are caught in a dreamless sleep, we would better be dead already.