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Some people swear that if they want to wake up at 6 am, they just bang their head on the pillow six times before going to sleep. Crazy? Maybe not. A study from 1999 shows that it all comes down to some nifty unconscious processing.

For three nights, a team at the University of Lubeck in Germany put 15 volunteers to bed at midnight. The team either told the participants they would wake them at 9 am and did, or told them they would wake them at 9 am, but actually woke them at 6 am, or said .

This last group had a measurable rise in the stress hormone adrenocorticotropin from 4.30 am, peaking around 6 am. People woken unexpectedly at 6 am had no such spike. The unconscious mind, the researchers concluded, can not only keep track of time while we sleep but also set a biological alarm to jump-start the waking process. The pillow ritual might help set that alarm.

The sleeping brain can also process language. In a 2014 study, Sid Kouider of the 脡cole Normale Sup茅rieure in Paris and his colleagues trained volunteers to push a button with their left or right hand to indicate whether they heard the name of an animal or object as they fell asleep. The team monitored the brain鈥檚 electrical activity during training and when the people heard the same words when asleep. Even when asleep, activity continued in the brain鈥檚 motor regions, indicating that the sleepers were preparing to push the correct button. The people could also correctly categorise new words, first heard after they had dropped off, showing that they were genuinely analysing the meaning of the words while asleep.

It鈥檚 an ability that makes good evolutionary sense, says Kouider. 鈥淚f you stop monitoring your environment, you become very vulnerable during sleep鈥� It makes sense that you don鈥檛 simply shut down, but continue tracking in a kind of standby mode.鈥� This might explain why some sounds, like our names, wake us more easily than others.

This protective monitoring may not last all night, however. A study published this year continues in REM sleep for words heard just before bed, once in deep sleep all responses disappear as the brain goes 鈥渙ffline鈥� to allow the day鈥檚 memories to be processed. 鈥淵our cognition about things in the environment declines progressively towards deep sleep,鈥� Kouider says. 鈥淪leep is not all-or-none in terms of cognition, it鈥檚 all-or-none in terms of consciousness.鈥�

This article appeared in print under the headline 鈥�1. Think while you sleep鈥�