Why do some muscles become paralysed during sleep?

THAT'S THE WHY:WHEN YOU are asleep, you cycle through different phases, including one called rapid eye movement or REM sleep. This is the phase where you typically have the dreams you later recall.

Did you know though that in normal REM sleep, your eyes may be moving but many muscles in other parts of your body don’t act like they do when you are awake? Why is that?

The muscles that go offline in normal REM sleep are skeletal muscles, which are involved in moving parts of your body like your arms and legs, rather than other muscle types that keep the heart or other vital processes chugging along.

One theory is that these skeletal muscles go into a temporary state of paralysis during REM sleep to literally stop us acting out movements from our dreams.

Quite how that happens at a biochemical level is still a matter of debate, and a new study on a rat model suggests that multiple brain chemicals and receptors could be involved.

The research, carried out at the University of Toronto, found that chemicals called GABA and glycine shut off motor neurons during REM sleep and appear to trigger REM paralysis, according to researcher John Peever from the University of Toronto in a press release about the findings.

“But we also identified the way cells detect GABA and glycine,” he adds.

“Motor neurons, like all brain cells, listen to these transmitters through receptors and we identified the receptors that allow GABA and glycine to shut the motor neurons off – the three different types of receptors that are required.”

The study was published this month in The Journal of Neuroscience.