Your brain hides memories of the things you hear while you're asleep

If monsters whisper in your ear while you sleep, you'll likely have no idea in the morning.

But your brain will, turning their words over somewhere deep in your mind, far beneath the surface of what your conscious thoughts can access.

That's the lesson of a paper out of PSL Research University in Paris.

A pair of researchers had 22 French speakers, aged 20 to 28, perform a series of tasks to test their sleep memories while hooked up to an EEG, which measures electrical activity in the brain (also known as brain waves).

First, while the participants (mostly women) were awake, the researchers played a mix of real French words and fake French-sounding words and instructed the subjects to press a button in one hand if the word was real and in the other if it was fake. (That was to see if they were actually paying attention and to make it harder to memorize what they were hearing.)

Later, they had the subjects perform the same task while drifting off to sleep in comfortable chairs in a dark room - a task many people can still do reasonably well in early stages of sleep. (If you're wondering if you're the only person who would find this procedure a little unsettling, you're not alone.)

There's significant evidence that parts of their minds were responding to and remembering the words heard while sleeping, even if their conscious minds couldn't call up the memories.

The researchers measured the subjects' level of wakefulness as they performed the task. Then, after they woke up, the researchers had them complete a memory test, going through a list of words (both real and fake) they'd heard while awake and while asleep.

Researchers found that - for the most part - the subjects could not consciously distinguish between words they'd heard while asleep and words they'd never heard before.

But when the participants said they'd never heard a word before and they'd actually heard it while asleep, they were not at all confident in their answers. That suggests that somewhere, deep in their brains, they may have had the faintest, vaguest recollection that those words weren't new at all.

Further, when they did successfully identify a word they'd heard while sleeping, they were very confident in their answers. And their EEG readings when presented with words heard in their sleep matched those of people remembering things - though not perfectly.

All of this suggests that that parts of their minds were responding to and remembering the words heard while sleeping, even if their conscious minds couldn't call up the memories.

The researchers write that this study offers evidence for the widely-believed but difficult-to-prove idea in cognitive science that there are two types of memory: "explicit episodic memory (i.e. 'remembering') and an implicit sense of familiarity (i.e. 'knowing')."

So while your brain turns off episodic memory while you sleep - perhaps because sleep plays some role in organizing the episodic memories from your day - it seems to continue to form implicit knowledge memories through some hidden mechanism.

Further, they write, it suggests that with some refinement, science could figure out how to help people learn while sleeping.

Now, this study used a small homogeneous sample. We'd have to see it replicated in several labs with a range of populations to become more confidant in its conclusions. But it's an exciting and promising early investigation of memory formation during sleep, and it's exciting to see where it goes.