It’s easy to imagine that as we drift off to sleep each night, we are powering down our brains and letting it get a moment’s rest before we’re back at it in the morning. That’s not exactly correct – instead of shutting off, our brains are actually pretty active while we sleep. They’re working on things like preserving your memories or storing information that wasn’t locked in from earlier in the day.
While we’re snoozing, our brains enter a sleep cycle that is broken up into two different parts: Non-REM (or NREM) sleep that has four specific stages and a Rapid Eye Movement (REM) cycle. So what exactly is happening during each of these cycles and why are they important? We’ve broken down each sleep stage for you and how it affects your body.
- Stage 1: This is the lightest stage of sleep and starts minutes (or even seconds) after people close their eyes. According to the National Sleep Foundation, it’s during this stage that the brain emits alpha and theta waves and one’s eye movement slows down. Stage 1 lasts for about seven minutes and people can easily be woken during this time.
- Stage 2: This is a slightly deeper stage of sleep, where it’s harder to wake up (or be woken up). There is a brief spike in brain wave frequency (known as “sleep spindles”) but then it slows down and the body prepares for sleep. During this time one’s body temperature will drop and their heart rate will slow.
- Stages 3 and 4: These final two stages of Non-REM sleep are the deepest and hardest to wake people up from. The National Sleep Foundation says this is when one’s body, “repairs muscle, and tissues, stimulates growth and development, boosts immune function, and builds up energy for the next day.” The last stages of NREM sleep are sometimes called “slow-wave,” “delta,” or “deep” sleep.
REM (Rapid Eye Movement) sleep occurs about 70 to 90 minutes after people initially fall asleep and makes up about 25 percent of their sleep cycle. REM sleep is (not surprisingly) characterized by rapid eye movement and is the sleep stage most associated with dreaming, according to the Amerian Sleep Association. One’s heart rate and blood pressure increase, and breathing become fast, irregular, and shallow, says the National Sleep Foundation.
“Interestingly, during REM sleep muscles in the arms and legs are temporarily paralyzed. This is thought to be a neurological barrier that prevents us from ‘acting out’ our dreams,” says researchers from Harvard Medical School’s Healthy Sleep website.
REM sleep has important health consequences. The National Sleep Foundation says that it impacts learning and memory function since it’s the portion of the sleep cycle when your brain consolidates and processes information from the day before so that it can be stored in your long-term memory.
If REM sleep is disturbed, it’s harder to make new memories or learn new skills. There is also some research to suggest that interrupted REM sleep is linked to dementia or other negative health outcomes.
Sleep Cycle Length & Pattern
Our sleep cycles will vary in length depending on our current age. On a regular night, it will follow a sequential order. However, there are some events that can shift how we get NREM and REM sleep. Here are some examples of factors that can change our sleep pattern, from Harvard Medical School’s Healthy Sleep website
- Age: Did you know that for the first year of our lives, sleep usually starts in the REM cycle? This most likely develops into the adult sleep cycle as our brain’s structure and function change.
- Sleep history: Our cycle can be affected by the type of sleep we have had recently. Staying up overnight or an irregular sleep schedule can shift when and how long our NREM or REM sleep occurs.
- Drug or alcohol use: “Alcohol before sleep tends to suppress REM sleep early in the night. As the alcohol is metabolized later in the night, REM sleep rebounds,” according to the Harvard Medical School team of researchers.
How Napping Affects Our Sleep Cycle
According to the National Sleep Foundation, people are going to want to keep their naps to about 20 minutes or stretch it out for 90 minutes (until they’ve gotten through one complete sleep cycle).
“Nap for 30 to 60 minutes and you’ll hit the deeper stages of sleep, where your brain waves slow down, making you feel groggy (as if you have a sleep hangover) when you wake up (also known as sleep inertia), “says the foundation. “It might not be worth it to nap at all if you’re going to nap for this amount of time because you’ll likely come out of your shuteye feeling less alert than before.”
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