What Is Core Sleep?

What you need to know about core sleep:

• Core sleep is the term Apple Watch uses for the lighter N1 and N2 stages of non-REM sleep, which form a normal part of your sleep cycle.
• There’s no set amount of core sleep you need each night, with sleep patterns naturally varying from person to person and night to night.
• A lower core sleep reading doesn’t necessarily mean there’s a problem, as how rested you feel can also be influenced by your overall sleep quality, sleep duration, and factors like stress.
• Instead of focusing on one sleep stage, getting enough sleep overall and maintaining a reasonably consistent sleep routine can help support better sleep.

If you wear a smartwatch or use a sleep tracking app, you may have woken up to a breakdown of your sleep. Alongside deep and REM sleep, you might see another category called core sleep. 

The term core sleep can be a little confusing, especially because it’s not used consistently when talking about sleep. Apple Watch, for example, uses core sleep as a way to describe the lighter stages of sleep, also called the N1 and N2 stages of non-REM sleep. 

In this guide, we’ll unpack what core sleep means and how it compares to the other stages of sleep.

What does core sleep mean?

Core sleep is a term typically used by some sleep trackers to describe the lighter, earlier stages of sleep. On Apple Watches, core sleep refers to N1 and N2 sleep1. 

N1 is the lightest stage of sleep, making up a small part of the night. N2 is longer and can account for around half your night’s sleep1. Throughout the night, your body moves through these different sleep stages, as well as deep sleep and REM sleep².

Where the meaning of core sleep gets confusing is with clinical science. In older sleep research, core sleep was actually used to describe slow-wave sleep3. However, now with Apple Watches, it specifically refers to the N1 and N2 stages of sleep1.

So, is core sleep good? Core sleep is a normal part of your sleep cycle, not a stage you need to maximise or minimise. Your sleep moves through different stages across the night, with lighter non-REM, deep and REM sleep each forming part of the overall pattern².

Core sleep vs deep sleep

The main difference between core and deep sleep comes down to where they sit within non-REM sleep. On Apple Watch, core covers N1 and N2, while Deep refers to N3, also known as slow-wave sleep1.

Core sleep is lighter and is part of the transition between sleep stages. Deep sleep is a deeper stage, characterised by slow-wave brain activity and has been linked with processes involved in physical recovery4. 

Core sleep vs REM sleep

Like deep sleep, core sleep and REM sleep are two different parts of the sleep cycle. Core sleep is the N1 and N2 stages of non-REM sleep, while REM sleep is a separate stage marked by rapid eye movements and changes in brain activity 1 2 . REM sleep is also related to dreaming and is involved in processes related to memory and emotional information5 . 

Signs you might need more core sleep 

While your sleep tracker might show less core sleep than usual, this means it has estimated less time spent in the lighter N1 and N2 stages of sleep. How rested you feel, however, can be influenced by different factors, including your overall sleep quality and how long you slept. 

Some signs include: 

• Feeling tired even after spending ‘enough’ time in bed 
• Waking frequently throughout the night
• Daytime fatigue or sleepiness
• Finding it harder to concentrate
• Waking up feeling sluggish or unrested

Research has shown that non-restorative sleep has been linked to fatigue and daytime sleepiness. It’s also found that performance is poorer on tests of cognitive function6.  

Daytime sleepiness and fatigue can also be influenced by sleep quantity and quality7, as well as other factors like stress8. 

Rather than treating your core sleep number as the cause, it's important to look at other things that may be impacting your sleep. Are you particularly stressed? Has your routine changed? Sometimes these can play a role in how rested you feel. 

How much core sleep do you need? 

If you’re wondering about how much core sleep you should get a night, it's important to look past one single number. There isn’t a set amount of core sleep that everyone needs each night. Sleep needs vary from person to person, and factors like health, lifestyle, and your sleep environment can all impact your sleep quality9 10. 

Another key factor is how regular your sleep and wake times are. Big differences in waking times have been linked to poorer sleep outcomes11. Sleep patterns can also shift night-to-night, depending on stress levels or recovery from illness12. 

It's typically recommended that adults aged 18 to 60 should get at least seven hours of sleep every night13. That recommendation is for total sleep, not a particular amount of core, deep, or REM. Rather than trying to increase one stage based on your sleep data, focus on getting enough sleep overall, keeping your sleep routine reasonably consistent, and paying attention to how rested you feel during the day.

The role of core sleep in overall wellbeing 

Core sleep is just one part of a much bigger sleep picture. Throughout the night, your body moves through lighter non-REM sleep, including the N1 and N2 stages that make up core sleep on Apple Watches. 

While sleep trackers can give you interesting insights into your sleep patterns, try not to get too caught up on the exact number. Instead, think of it as a guide. Looking at patterns over time, alongside how you feel, can give you a better picture of your overall sleep. 

Want to learn more about what happens when you go to sleep each night? Read more articles like the importance of sleep and what sleep debt is and how to pay it off on the Swisse Wellness Hub. 

References:

1. Apple. Estimating Sleep Stages from Apple Watch. 2025. 
2. Brinkman JE, Sharma S, Reddy V. Physiology of Sleep. StatPearls Publishing. 2023. 
3. Wauquier A, van Sweden B. Aging of core and optional sleep. Biological Psychiatry. 1992;31(9):866–880. doi:10.1016/0006-3223(92)90114-F. 
4. Dijk DJ. Regulation and functional correlates of slow wave sleep. Journal of Clinical Sleep Medicine. 2009;5(2 Suppl)–S15 
5. Perogamvros L, Dang-Vu TT, Desseilles M, Schwartz S. Sleep and dreaming are for important matters. Frontiers in Psychology. 2013;4:474. doi:10.3389/fpsyg.2013.00474 
6. Tinajero R, Williams PG, Cribbet MR, Rau HK, Bride DL, Suchy Y. Nonrestorative sleep in healthy, young adults without insomnia: Associations with executive functioning, fatigue, and pre-sleep arousal. Sleep Health. 2018;4(3):284–291. doi:10.1016/j.sleh.2018.02.006 
7. Slater G, Steier J. Excessive daytime sleepiness in sleep disorders. Journal of Thoracic Disease. 2012;4(6):608–616. doi:10.3978/j.issn.2072-1439.2012.10.07 
8. Kim EJ, Dimsdale JE. The effect of psychosocial stress on sleep: a review of polysomnographic evidence. Behavioral Sleep Medicine. 2007;5(4):256–278. doi:10.1080/15402000701557383 
9. Kohyama J. Which Is More Important for Health: Sleep Quantity or Sleep Quality? Children (Basel). 2021;8(7):542. doi:10.3390/children8070542 
10. Chaput JP, Dutil C, Sampasa-Kanyinga H. Sleeping hours: what is the ideal number and how does age impact this? Nature and Science of Sleep. 2018;10:421–430. doi:10.2147/NSS.S163071 
11. Chaput JP, Dutil C, Featherstone R, et al. Sleep timing, sleep consistency, and health in adults: a systematic review. Applied Physiology, Nutrition, and Metabolism. 2020;45(10 Suppl 2)–S247. doi:10.1139/apnm-2020-0032 
12. Yu X, Nollet M, Franks NP, Wisden W. Sleep and the recovery from stress. Neuron. 2025. doi:10.1016/j.neuron.2025.04.028 
13. Liu Y, Wheaton AG, Chapman DP, Cunningham TJ, Lu Hsi, Croft JB. Prevalence of Healthy Sleep Duration among Adults—United States, 2014. MMWR Morbidity and Mortality Weekly Report. 2016;65(6):137–141. doi:10.15585/mmwr.mm6506a1.