We’ve written before at HuffPost UK about how much rapid eye movement, or REM, sleep you need per night.

But to be honest, I hadn’t even heard of the term “core” sleep until recently.

Core sleep, also known as “slow wave” sleep, is “the most essential part” of your slumber, a sleep study says. It’s made up of a combination of phases of sleep that happen at the middle and end of the sleep cycle.

Speaking to The Telegraph, Dr Guy Meadows, the founder and clinical director of The Sleep School and creator of the Sleep School App, said: “Core sleep includes the most restorative stages of sleep,........

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