Apple has never been shy about inventing its own vocabulary for technical terms, and some of the Apple Watch’s tracking metrics are no exception. Case in point: basic sleep. It’s a term Apple uses in its Apple Watch and Health apps that refers to a stage of sleep, specifically light sleep, where your body is asleep but not yet in the deeper, more restorative phases of rest. Deep sleep, by Apple’s definition, is the early non-REM stage, called the N1-N2 stage by sleep scientists.
It’s not abnormal to see a lot of baseline sleep in your Health app, as it is the longest part of most sleep cycles and often makes up the majority of nighttime rest. Basically, deep sleep is the period when your heart rate and breathing slow down, your body relaxes, and you can still wake up more easily than during deep sleep.
How Core Sleep Fits into Your Sleep Cycle
Apple Watches, the Health app, and many of the best wearables for sleep tracking divide sleep into four stages: awake, deep, deep, and REM. Core sleep tends to be the dominant phase in terms of timing, so don’t be alarmed if you notice periods of core sleep that are much longer than REM or deep sleep.
Although deep sleep may not be as restorative as the other phases, it is still absolutely vital to your body’s nighttime rhythms. It’s a sort of scaffolding that supports the structure of your sleep schedule. Although not as critical for memory processing or dreaming as REM sleep, it is still vital for brain and body recovery and, more importantly, for the transition from wakefulness to these deeper stages.
Apple chose the term “core” because it was concerned that calling it “light” sleep would trivialize what is truly a very important phase. In a technical document on how the Apple Watch estimates sleep stages, the tech giant points out that the main stage contains “sleep spindles and K complexes.” Sleep spindles are brief bursts of rapid brain activity that help stabilize sleep and process memories, while K complexes are large, slow brain waves that help the brain decide whether to stay asleep or wake up in response to stimuli.
What do the other stages of sleep mean
Deep sleep is the phase most closely linked to recovery. This includes physical recovery and general restoration as well as supporting your body’s immune system. It tends to appear in much shorter blocks than core sleep and is more difficult to wake from. A number of causes can lead to a lack of deep sleep, including alcohol consumption, stress, or changes in your schedule. There is also natural variation independent of these external triggers.
REM sleep is the stage most linked to vivid dreams, emotional regulation, and the processing of memories. Longer phases of REM sleep tend to appear later in the night, although you will typically go through a few phases of REM sleep during a single night of sleep. In general, the most useful way to look at your sleep data is to cover a number of full nights of sleep, rather than focusing too much on any one stage in isolation. If you’re having trouble sleeping in general, consider these expert tips for a better night’s sleep, or perhaps invest in a high-quality noise machine.
