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, primarily deep sleep (non-REM, stage three) and REM sleep, which are crucial for physical and cognitive health.”

How can I tell if I’m getting enough?

Core sleep is key for memory consolidation, emotional processing, immune function, and muscle repair. 

It’s even been linked to a lower dementia risk. 

Some signs you’re not getting enough core sleep include, per The Sleep Foundation: 

• Feeling unrefreshed and drowsy
• Reduced alertness and attention
• Trouble learning and forming new memories
• Cravings for high-calorie food. 

 GP Dr Anatalia Moore told Glamour that it can also give you dark circles under your eyes and sallow skin, because your body hasn’t had enough time to repair itself. 

“If this continues the range of symptoms is phenomenal, ranging from low mood, difficulty concentrating and low libido, to weight gain, diabetes, poor immunity and hormonal issues,” she added. 

So how many hours of core sleep do I really need?

Everyone’s different ― core sleep usually makes up about half of your overall sleep, but because it happens at the tail end of different sleep cycles, it can be hard to work out how much of your sleep is the “most essential” kind.

And of course REM sleep is crucial to good rest too. 

So rather than trying to work out how much you can shave off your sleeping hours, it might be better to try to get enough kip overall (the NHS puts it at between seven to nine hours).

They say you may have a sleep problem if: 

• find it difficult to fall asleep
• lie awake for long periods at night
• wake up several times during the night
• wake up early and be unable to get back to sleep
• feel down or have a lower mood
• have difficulty concentrating
• be more irritable than usual.

If you notice these signs, speak to your doctor.