Everyone dreams – even people who believe that they “never dream” and can’t remember any of their dreams. That’s according to a group of French researchers writing in the Journal of  Research. In questionnaire surveys, up to 6.5% of people report that they ‘never dream’. Although most of these people report having dreamed at some point in the past, roughly 1 in every 250 people say that they can’t remember ever dreaming – not even once.

But is it possible that these “non-dreamers” do in fact have dreams, but just can’t remember them?

To study this question, Herlin et al., authors of the new paper, looked at people with REM sleep behavior disorder RBD, a condition in which patients make movements, sometimes violent ones, while they sleep. Sometimes these movements are accompanied by speech. The movements seen in RBD are often quite complex and, interestingly, they seem to correspond to the content of the dreams that patients are experiencing. At least some of the actions seen in RBD are ‘acted out’ dreams.

Herlin et al. report that some RBD patients report never dreaming: out of 289 diagnosed RBD cases, 2.7% reported that they had not dreamed for at least 10 years, and 1.1% said that they had never dreamed ever. However, in many cases their actions during sleep (captured on video in the sleep clinic) suggested that they were dreaming. Herlin et al. quote some examples: This certainly sounds like dream-like speech and actions. On the basis of these cases, Herlin et al. conclude that “dreaming production is universal, while dreaming recall is variable.”

Which raises an interesting philosophical question: what is a dream? Is it simply a subjective experience during sleep? If so, how can be sure that these patients are dreaming? Maybe they are just behaving as if they are dreaming, but without any conscious content. Maybe non-dreamers are a concrete example of philosophical zombies, a zombies of hypothetical creature that behaves like a normal human, but has no conscious experiences.

On the other hand, maybe some people do have dreams but never remember them. Maybe we all experience this – we might have many dreams every night, and only remember some of them. But are they really “my” dreams if I don’t remember them? Or is memory what binds together experiences into my identity?

I do note, however, that there is a selection bias in this study. All of the patients were undergoing evaluation for sleep behavioral problems, which seems to mean that they were making movements in their sleep that were a risk to themselves or others.

However, if the movements in RBD are related to dreams, then someone who really never dreams would never make such movements. Such a person could develop full-blown RBD, but no-one would ever know about it, because they would have no dreams to act out. In other words, genuine non-dreamers might be out there, and would never make it into a study like this.