Friends, let's talk about one of the most confusing terms youll see on your fitness trackerspecifically your Apple Watch. Next to REM sleep, which youve probably heard of, and deep sleep, which feels self explanatory, theres core sleep. And if you google what core sleep means, youll get a definition that is entirely opposite from how Apple uses the term. So lets break it down.The root of the confusion lies in the fact that the term core sleep has been used in the scientific literature to mean a few different things. Importantly, its not a recognized sleep stage. Apple, on the other hand, decided to rename the sleep stages its watch can detect, and called one of them core sleepbut it bears no relation to any of the previous common uses of the term."Core sleep" in the Apple Watch is the same as light sleepLet me give you a straightforward explanation of what youre seeing when you look at your Apple sleep data.Your Apple Watch tries to guess, mainly through your movements, when youre in each stage of sleep. (To truly know your sleep stages would require a sleep study with more sophisticated equipment, like an electroencephalogram. The watch is just doing its best with the data it has.)Apple says their watch can tell the difference between four different states:AwakeLight (core) sleepDeep sleepREM sleepThese categories roughly correspond to the sleep stages that neuroscientists can observe with polysomnography, which involves hooking you up to an electroencephalogram, or EEG. (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. That means stages 1 and 2, which are sometimes called light sleep, are being labeled as core sleep by your wearable.In other words: Apple's definition of "core sleep" is identical to scientists' definition of "light 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? The company says in a document on their sleep stage algorithm that they were worried people would misunderstand the term "light sleep" if they called it that.The label Core was chosen to avoid possible unintended implications of the term light, because the N2 stage is predominant (often making up more than 50 percent of a nights sleep), normal, and an important aspect of sleep physiology, containing sleep spindles and K-complexes.In other words, they thought we might assume that "light" sleep is less important than "deep" sleep, so they chose a new, important-sounding name to use in place of "light." A chart on the same page lays it out: non-REM stages 1 and 2 fall under the Apple category of core sleep, while stage 3 is deep sleep. Thats how Apple defined it in testing: If an EEG said a person was in stage 2 when the watch said they were in core, that was counted as a success for the algorithm.What are the known sleep stages, and where does core sleep fit in?Lets back up to consider what was known about sleep stages before Apple started renaming them. The current scientific understanding, which is based on brain wave patterns that can be read with an EEG, includes these stages:Non-REM stage 1 (N1)N1 only lasts a few minutes. Youre breathing normally. Your body is beginning to relax, and your brain waves start to look different than they do when youre awake. This would be considered part of your light sleep. The Apple Watch considers this to be part of your core sleep stage.Non-REM stage 2 (N2)Also usually considered light sleep, N2 makes up about half of your sleep time. This stage includes spikes of brain activity called sleep spindles, and distinctive brainwave patterns called K 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.Non-REM stage 3 (N3)N3 is often called deep sleep, and this stage accounts for about a quarter of your night. It has the slowest brain waves, so its sometimes called slow wave sleep. Its hard to wake someone up from this stage, and if you succeed, theyll be groggy for a little while afterward. This is the stage where the most body repair tends to happen, including muscle recovery, bone growth in children, and immune system strengthening. As we age, we spend less time in N3 and more time in N2.(There was an older classification that split off the deepest sleep into its own stage, calling it non-REM stage 4, but currently that deepest portion is just considered part of stage 3.)REM sleepREM 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).The brain waves of a person in REM sleep look very similar to those of a person who is awake, which is why some sleep-tracking apps show blocks of REM as occurring near the top of the graph, near wakefulness. We dont usually enter REM sleep until weve been through the other stages, and we cycle through these stages all night. 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? Using Apple's definition, in which core sleep is the same as light sleep, it's normal for almost half of your sleep to be core sleep. Sleep scientists give an approximate breakdown (although the exact numbers may vary from person to person, and your needs aren't always the same every night):N1 (very light sleep): About 5% of the total (just a few minutes)N2 (light or "core" sleep): About 45%, so just under four hours if you normally sleep for eight hoursN3 (deep sleep): About 25%, so about two hours if you normally sleep for eight hoursREM: About 25%, so also about two hours.How to get more core sleepIf your Apple watch says you're getting less core sleep than what we mention above, you might wonder how you can get more core (or light) sleep. Before you take any action, though, you should know that wearables aren't very good at telling exactly what stage of sleep you are in. They're usually (but not always!) pretty good at telling when you are asleep versus awake, so they can be useful for knowing whether you slept six hours or eight. 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. That said, the best way to get more core sleep is to get more and better sleep in general. Start with this basic sleep hygiene checklist. Among the most important items: Give yourself a bedtime routine with at least 30 minutes of wind-down time where you try to do something relaxing.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 improve all your sleep stages, whether your Apple Watch can tell them apart or not.Other ways people use the term core sleepI really wish Apple had chosen another term, because the phrase core sleep has been used in other ways. It either doesnt refer to a sleep stage at all, or if it is associated with sleep stages, its used to refer to deep sleep stages.In the 1980s, sleep scientist James Horne proposed that your first few sleep cycles (taking up maybe the first five hours of the night) constitute the core sleep we all need to function. The rest of the night is optional sleep, which ideally wed still get every night, but which its not a big deal to miss out on from time to time. He described this in a 1988 book called Why We Sleep (no relation to the 2017 book by another author) but you can see his earlier paper on the topic here. 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 everything but light sleep. For example, this paper on how sleep changes as we age compares their findings in terms of sleep stages with Hornes definition of core sleep. In doing so, they describe core sleep as mainly consisting of stages N3-N4 (in other words, N3 as described above).From there, somehow the internet has gotten the idea that N3 and REM are considered core sleep. I dont know how that happened, and I dont see it when I search the scientific literature. I do see it on what is core sleep? junk articles on the websites of companies selling weighted blankets and melatonin gummies.For one final, contradictory definition, the phrase core sleep is also used by people who are into polyphasic sleep. This is the idea that you can replace a full nights sleep with several naps during the day, something that biohacker types keep trying to make happen, even though it never pans out. They use the term pretty straightforwardly: If you have a nighttime nap that is longer than your other naps, thats your core sleep. Honestly, thats a fair use of the word. I'll allow it.So, to wrap up: Core sleep, if youre a napper, is the longest block of sleep you get during a day. Core sleep, to scientists who study sleep deprivation, is a hypothesis about which part of a nights sleep is the most important. But if youre just here because you were wondering what your Apple sleep app means by "core sleep," it means stages N1-N2, or light sleep.