Dreams.

Let’s start with what we know. First: everyone dreams. Even if they say they don’t, they do, somewhere between four and six times a night. Children under the age of ten dream even more than that; babies in particular spend an extra 50–80 per cent of their sleeping time in the REM (rapid eye movement) phase, where most dreaming occurs. But it’s not just humans who dream: all mammals, some birds and reptiles—and perhaps even squid—experience REM sleep too. Dreaming seems to be one of the preconditions of complex brain construction.

So, why do we dream? Well, that’s where it gets complicated. “For most of human history, dreaming was thought of as a portal into a separate existence,” says Drew Dawson, a sleep researcher at Central Queensland University. “Dreams were a twisted version of the waking world that could afford us insights into our own reality.” Animist cultures, such as