Scientists once considered sleep to be like a shadow (阴影) getting drawn over a window between the brain and the outside world: when the shadow is closed, the brain stops reacting(反应) to outside stimuli(刺激).

A study published on 12 October in Nature Neuroscience suggests that there might be periods during sleep when that shadow is partially open.

A few years ago, Oudiette, a cognitive(认知) scientist in France, began questioning the definition of sleep after she and her team conducted an experiment in which they were able to communicate with people who are aware that they are dreaming while they sleep otherwise known as. clear dreamers. During these people's dreams, experimenters were able to ask questions and get reactions through eye and facial-muscle movements.

Karen Konkoly, who was a co-author on that study, says "it was a big open question in our mind whether communication would be possible with non-clear dreamers."

So Oudiette continued with the work. In her latest study, she and her colleagues observed 27 people with narcolepsy (嗜睡症)—characterized by daytime sleepiness and a high frequency of clear dreams——and 22 people without the condition. While they were sleeping, participants(参与者) were repeatedly asked to frown or smile. All of them reacted correctly to at least 70% of these tips. Overall response rates(整体反应率) were higher -for all participants during REM (rapid eye movement) sleep, when the deepest sleep occurs but the brain remains quite active, than during other sleep stages.

This study is part of a larger advance in the field of sleep research, says Mélanie Strauss, a cognitive scientist in Brussels. Researchers are moving away from monitoring sleep mainly with EEG(脑电图)and towards "fine grained"(细粒度 approaches that combine EEG with various tasks and stimuli a strategy that could help to throw light. on specific diseases or conditions, she says.