How do you realize you're dreaming while asleep?

You realize you're dreaming when a thinking part of your brain that usually goes quiet during REM sleep flickers back on. A lucid dream is simply a dream in which you know you are dreaming. Lucid dreams begin in REM sleep, the stage where most vivid dreams happen, and when researchers look inside the brain during one, they see extra activity in the dorsolateral prefrontal cortex, a region tied to working memory, planning and self-consciousness. Normally it is one of the few areas switched off in REM, which may be why ordinary dreams rarely make you stop and ask what's going on.

The best proof came from a clever trick. In 1975 the psychologist Keith Hearne realized that while the body is almost paralysed in REM, the eyes still move. He agreed a signal with an experienced lucid dreamer, Alan Worsley: once you know you're dreaming, move your eyes in a set pattern. At around 8 am on 12 April 1975, Hearne's instruments recorded that signal, sent from inside a dream. Today, teams of scientists hold real-time two-way conversations with lucid dreamers through eye movements and facial muscle signals.

It's more common than you might think: in a pooled sample of more than 24,000 people, 55% said they had had at least one lucid dream. And it can be learned. One classic method is to keep suspecting, during the day, that you might be dreaming, so that the habit follows you into sleep.