Most of us know that a good night’s rest can make all the difference in how we feel the next day. But those who wear a sleep tracker might wake up feeling refreshed, then be puzzled by the results. For example, what does it mean that you spent only four hours in “core sleep” the night before? And is that something you really need to be worried about?
Sleep moves through lighter, deeper and REM stages throughout the night, and “each stage of sleep is vitally important,” says Rebecca Robbins, a sleep scientist at Brigham and Women’s Hospital and an assistant professor at Harvard Medical School.
But a wearable tracker can’t always tell the whole story of how well you slept the night before. And some devices group medically recognized stages together
under their own labels.
Here’s what to know about the different stages of sleep, what “core sleep” actually means and how much weight to give that number on your screen.
What are the different stages of sleep? How important is core sleep?
Sleep comes in cycles, with no single “best” stage, says Robbins. Stage 1 is the lightest stage of non-rapid eye movement sleep, which is why it's also known as N1. It’s the transition into sleep, when your body begins relaxing and your brain waves start slowing down.
"A well-rested person typically takes approximately 15 minutes to fall asleep,” Robbins says.
Stage 2, or N2, is a more advanced stage of light sleep, when your body temperature drops and heart rate slows.
“N2 usually takes up the biggest portion of sleep each night − around half of your total sleep,” says Dr. Raj Dasgupta, a pulmonary and sleep specialist at Huntington Memorial Hospital in California.
Stage 3, or N3, is deep, restorative, slow-wave sleep. It's the stage that supports tissue repair and immune function, and is the hardest stage to wake up from, says Dasgupta.
The fourth stage, REM sleep, is characterized by rapid eye movements, high brain activity, dreaming and temporary muscle paralysis.
Usually, we cycle through these four sleep stages multiple times throughout the night, according to the National Institutes of Health.
"Each stage serves a vitally important role − supporting our next day and future health, performance and well-being," says Robbins.
Indeed, too little sleep or frequently interrupted sleep can leave you less alert the next day and increase long-term health risks. At the same time, Dasgupta cautions that how much time you spend in any one sleep stage cannot tell you how healthy you actually are.
What is core sleep? How long should you be in core sleep?
While core sleep "is not an official medical sleep stage,” says Dasgupta, it’s a label some wearable sleep trackers use for the lighter stages of non-REM sleep, namely N1 and especially N2. Apple, for example, groups N1 and N2 stages under “Core" sleep.
But Robbins calls the "Core" sleep term “an artifact of sleep tracking device manufacturer jargon that's making its way into the vernacular” even though its data may not be as reliable as some wearers assume.
For example, in a sleep lab, specialists identify N1, N2, N3 and REM sleep stages using recordings of brain activity, eye movements and other meaningful body signals. But most fitness and sleep trackers instead use wrist motion and an algorithm to estimate the stages. In other words, wearing one might help you spot patterns, but it cannot measure sleep stages the way a sleep study does.
In fact, because we spend “40% to 50% of our night” in light or "core" sleep anyway, Robbins says, spending a large part of the night in this sleep stage is normal, and not a sign that you missed out on “real” sleep, as trackers can sometimes make their wearers feel.
But that isn't to say core sleep, or N1 and N2 sleep, isn't important. "Light sleep provides essential benefits, such as aiding in memory processing,” Robbins says. And research also links getting enough N2 sleep to improved motor skills. Dasgupta also notes that light sleep is an essential stage to move through on your way to other stages, because without it, you wouldn’t progress through a normal night of rest or get the full benefit of all the sleep stages.
How to improve core sleep
Because light or "core" sleep is no more important than the other sleep stages, there’s no need to aim to get more or less of it. To instead give your body enough time to experience all the stages, first, be sure you're "making time for enough sleep overall,” Robbins says. Specifically, most adults need seven to nine hours nightly.
It's also important to “keep sleep consistent,” Robbins says. That means similar bedtimes and wake times, even on weekends. Dasgupta recommends morning light and regular exercise to support your sleep-wake rhythm. And Robbins suggests “a bedtime ritual you can look forward to." This could be a relaxing routine such as light reading, gentle stretching or a warm bath.
Dasgupta also suggests going easy on caffeine later in the day, plus limiting alcohol consumption and heavy meals near bedtime, as well as the light from a television, iPad or phone screen. Your sleep environment matters, too, as being comfortable through the night helps you stay asleep long enough to cycle through each stage. Aim for a "cool, dark, quiet bedroom," he adds.
If you routinely take more than 30 minutes to fall asleep once you’re in bed, Robbins recommends speaking with a health care provider. Persistent sleep problems such as loud snoring or daytime sleepiness also warrant a conversation.
And if you want to keep an eye on your sleep patterns in addition to such conversations, that's great, but just don’t let one night’s sleep data become a scorecard for your overall sleep.
"I would pay more attention to whether you are getting enough sleep, staying asleep reasonably well and feeling rested the next day," says Dasgupta, "rather than relying much on any 'core sleep' number."
This article originally appeared on USA TODAY: Is core sleep just 'light' sleep? Here’s why it matters
