It is the perfect learning shortcut, to play a language tape or revision recording at night while you are asleep.
But those desperately hoping the information will go in as they snooze may be disappointed.
Scientists have previously found that the brain does take in what it hears during REM sleep – the time spent mostly dreaming, usually in the morning before we wake up.
Leaving a tape running overnight is probably counter-productive as information gained in deep sleep can be completely lost.
Deep sleep
French researchers found that sound played during certain parts of deep sleep may make information harder to learn when you wake up than if you had never heard it before.
That is thought to be because the brain is busy erasing memories at this time, and the new knowledge is dumped along with them.
In a study published by experts from PSL Research University in Paris in August 2017, researchers tested sleep learning by playing 20 participants white noise, which contained patterns of sound.
REM stage
The sounds heard during the REM (rapid eye movement) stage of sleep were remembered by these people when they woke up.
They found it easier to identify the white noise which had repeated sounds in it because they had heard it while asleep.
But the noise played while people were in deep sleep, which makes up almost a third of our slumbers, was forgotten.
-Daily Mail.uk