Let’s talk about one of the most confusing terms youll see on your fitness trackerspecifically yourApple Watch.

So lets break it down.

It isnota key in of deep sleep, and has no relation to REM.

But in the scientific literature, “core sleep” is not a sleep stage at all.

It can refer to a portion of the night that includes both deep and light sleep stages.

There are a few other definitions, which I’ll go into below.

Your Apple Watch tries to guess, mainly through your movements, when youre in each stage of sleep.

The watch is just doing its best with the data it has.)

(Thats the thing where they attach wires to your head).

Scientists recognize three stages of non-REM sleep, with the third being described as deep sleep.

It is otherwise known as N2 sleep.

(More on that in a minute.)

So why didnt Apple use the same wording as everyone else?

What are the known sleep stages, and where does core sleep fit in?

Lets back up to consider what was known about sleep stagesbeforeApple started renaming them.

This would be considered part of your light sleep.

The Apple Watch considers this to be part of your core sleep stage.

This stage includes spikes of brain activity called sleep spindles, and distinctive brainwave patterns calledK complexes.

(These are what the Apple document mentioned above.)

This stage of sleep is thought to be when we consolidate our memories.

Fun fact: if you grind your teeth in your sleep, it will mostly be in this stage.

This stage makes up most of what Apple reports as your core sleep.

It has the slowest brain waves, so its sometimes called slow wave sleep.

As we age, we spend less time in N3 and more time in N2.

REM sleep is so named because this is where we have Rapid Eye Movement.

Your body is temporarily paralyzed, except for the eyes and your breathing muscles.

This is the stage best known for dreaming (although dreams can occur in other stages as well).

Usually REM sleep is fairly short during the beginning of the night, and gets longer with each cycle.

How much core sleep do I need?

They’re usually (but not always!)

But I wouldn’t make any changes to my routine based on the specific sleep stage numbers.

The algorithm can easily miscategorize some of your light sleep as deep sleep, or vice versa.

Start withthis basic sleep hygiene checklist.

Have a consistent wake-up time.

Don’t look at screens right before bed.

Keep your bedroom dark and cool.

Don’t have alcohol or caffeine in the evenings.

Improving your sleep overall will improveallyour sleep stages, whether your Apple Watch can tell them apart or not.

He uses the terms obligatory and facultative sleep in that paper, and switched to the core/optional terminology later.

Youll also find people using the phrase core sleep to refer to everythingbutlight sleep.

From there, somehow the internet has gotten the idea that N3andREM are considered core sleep.

I dont know how that happened, and I dont see it when I search the scientific literature.

Ihaveseen it on what is core sleep?

junk articles on the websites of companies selling weighted blankets and melatonin gummies.

Honestly, thats a fair use of the word.

I’ll allow it.