For a healthy adult sleeping about eight hours, you can expect roughly 50% of the night in core (light) sleep, 20% in deep sleep, and 20-25% in REM sleep. In practical terms, that works out to about four hours of core sleep, 60 to 100 minutes of deep sleep, and 90 to 120 minutes of REM sleep. The remaining time is spent in the lightest stage of sleep (N1), which makes up a small fraction as you drift off and during brief transitions between cycles.
If you’re here, you probably just looked at your sleep tracker and want to know whether your numbers are normal. Here’s what each stage does, what the targets actually mean, and why your tracker’s numbers deserve some skepticism.
What “Core Sleep” Actually Means
“Core sleep” is not a clinical term. It’s a label that Apple Watch and some other sleep trackers use for Stage 2 (N2) sleep, the lighter but stable phase where your heart rate slows and your body temperature drops. In a sleep lab, researchers simply call this light sleep. It makes up the largest chunk of your night because your brain cycles through it repeatedly between periods of deep and REM sleep.
Core sleep isn’t filler. During this stage, your brain produces short bursts of electrical activity called sleep spindles, which help transfer new information from short-term to long-term memory. It also serves as the scaffolding that holds your sleep cycles together. Without enough of it, you’d struggle to reach and sustain the deeper, more restorative stages. Four hours or so is typical, and most people don’t need to worry about getting too little of it unless their total sleep time is short.
How Much Deep Sleep You Need
Adults should aim for about 20% of total sleep in the deep stage, which comes out to roughly 60 to 100 minutes per night. Deep sleep is the hardest stage to wake from. Your brain produces large, slow electrical waves, your muscles fully relax, and your body shifts into repair mode.
During deep sleep, your body releases high levels of growth hormone, which drives tissue and muscle repair. Your brain also runs a waste-clearing process that flushes out metabolic byproducts, including proteins linked to dementia. This is the stage most responsible for feeling physically restored the next day. If you wake up feeling groggy and sore despite sleeping a full eight hours, low deep sleep is a common culprit.
Deep sleep is heavily front-loaded. You get most of it in the first half of the night, particularly in the first two sleep cycles. That’s why going to bed late but sleeping “enough hours” can still leave you feeling unrested: a delayed bedtime or alcohol before bed can cut into those critical early cycles.
How Much REM Sleep You Need
REM sleep should account for roughly 20-25% of the night, or about 90 to 120 minutes. This is the stage where most vivid dreaming occurs, your eyes move rapidly beneath your lids, and your brain is nearly as active as when you’re awake.
REM sleep plays a central role in consolidating emotional memories, procedural skills (like learning a musical instrument or a new athletic movement), and complex information. It’s also when your brain processes difficult emotional experiences, which is one reason poor sleep makes stress and anxiety feel so much worse.
REM sleep works on the opposite schedule from deep sleep. You get very little of it in your first sleep cycle, and it increases with each cycle through the night. Your longest REM periods happen in the final two to three hours of sleep. This is why people who cut their sleep short by waking up early disproportionately lose REM time, even if they still get decent deep sleep.
How Sleep Stages Shift With Age
Newborns spend about half their sleep in REM. By adulthood, that drops to around 20-25% and stays relatively stable through middle age. Deep sleep, on the other hand, declines steadily starting in your 30s. By your 60s and 70s, you may get half the deep sleep you got at 25, or even less. This is normal, though it does mean older adults often feel less physically restored by sleep and may benefit from prioritizing sleep hygiene to protect whatever deep sleep they still get.
Core sleep tends to fill in the gap as deep sleep declines, which is why older adults often see high percentages of light sleep on their trackers. This isn’t a sign of poor sleep quality on its own.
How Accurate Your Sleep Tracker Really Is
Consumer wearables are reasonably good at detecting whether you’re asleep or awake, but their ability to distinguish between specific sleep stages varies widely. A 2024 systematic review comparing popular trackers against polysomnography (the gold-standard sleep study) found significant differences between devices.
- Fitbit Charge 4 performed best, correctly identifying deep sleep about 75% of the time and REM sleep about 87% of the time. It slightly underestimated deep sleep by about 19 minutes per night on average.
- WHOOP hit roughly 65-67% accuracy for both deep and REM detection, but overestimated REM sleep by about 21 minutes per night.
- Garmin Vivosmart 4 struggled the most, correctly identifying deep sleep only 45% of the time and REM sleep just 34% of the time. It overestimated deep sleep by about 24 minutes per night.
The takeaway: use your tracker’s numbers as rough trends over weeks and months rather than treating any single night’s breakdown as precise. If your tracker consistently shows 30 minutes of deep sleep when you’d expect 60-100, that pattern is worth paying attention to. But a night-to-night fluctuation of 15 or 20 minutes is well within the device’s margin of error.
What Disrupts Your Sleep Stages
Alcohol is one of the most common disruptors of sleep architecture. It acts as a sedative that may help you fall asleep faster, but it suppresses REM sleep, particularly in the second half of the night. Over time, this creates a cycle: poor sleep from REM suppression leads to daytime fatigue, which leads to more self-medication with alcohol.
Caffeine, even consumed six hours before bed, can reduce total deep sleep and delay sleep onset. Because deep sleep is concentrated in the first few hours of the night, anything that delays your ability to fall into a stable sleep rhythm chips away at your most restorative stage first.
Exercise generally increases deep sleep, particularly if done earlier in the day. Consistency in your sleep and wake times matters too. Your brain’s sleep pressure system builds up throughout the day, and going to bed at a regular time allows it to allocate deep sleep and REM sleep in the predictable front-loaded and back-loaded pattern your body expects. Irregular schedules fragment this architecture even when total sleep time looks adequate.