Have you ever wondered what happens to your brain and body while you’re asleep?
Why is it that sometimes you wake up feeling fully rested, and other mornings it seems like you barely closed your eyes before the alarm buzzes?
And why do you vividly remember dreams only some of the time? Does that mean the rest of the nights you didn’t dream at all?
It turns out that the answers to these questions are found by studying the different stages of sleep we experience during the night. There are four in all (five if you count REM), and we pass through each of them multiple times every time we go to bed.
In this article, we’ll describe in detail what happens during each level, and also answer your most burning questions about your brain, the importance of REM (regarding sleeping, not the band), and what happens to these cycles as you age.
What is a Sleep Cycle?
A cycle is about 90 to 120 minutes in length and consists of all five stages, starting with light nap and progressing down into layers of deeper unconsciousness. The last is REM or Rapid Eye Movement. This is the part of the cycle where dreaming occurs.
While the stages do happen sequentially, they don’t go in a continuous loop. Rather, they reverse their order halfway through the cycle. For example, you’ll go through all the numbered stages (except REM), and then instead of circling back to the first, you’ll go backward.
So, after stage three or four, you’ll return to the second, and finally the first, characterized as the lightest phase. After that, you’ll have your first REM cycle. Then the process repeats all over again. That entire cycle takes at least 90 minutes.
The individual phases vary but typically last anywhere from five to fifteen minutes. At the beginning of the night, the cycles are shorter, about 90 minutes long. As duration continues, they lengthen to anywhere from 100 to 120 minutes. On average, people experience four or five of them per night.
Experts advise that to experience the most quality rest, especially if you get less than seven hours of shuteye per night, you should plan your waking time for at the beginning or end of a cycle when you’re in a light sleep. The verdict is that if you get woken up when you’re in one of the deeper stages, you’ll feel groggy and tired all day.
In Stage 1, you’re in a very light sleep. This is when you first doze off, and you can easily be woken or startled. This phase is very short, lasting anywhere from one to ten minutes. Here’s what you might expect during this part of the cycle:
- Slower breathing and a regular heartbeat.
- A decrease in blood pressure and brain temperature.
- Jerking and twitching, plus that frightening experience of feeling like you’re falling. This is apparently an evolutionary trait that prevented us from falling out of the trees we supposedly slept in. Here’s another fun fact: if you tend to keep irregular slumber habits, also known as poor sleep hygiene, you are more likely to experience a hypnic jerk.
- Eyes may roll back a bit or eyelids might remain partially open.
As you fall more deeply asleep, you’ll experience the following:
- Even slower heart rate and lower temperature than Stage 1.
- Other bodily processes also slow dramatically, including blood pressure and digestion.
- You become more difficult to wake up.
- Your brain waves become larger and more spread out.
This phase is still classified as light sleep and lasts approximately 20 minutes. But because the cycle circles back around, we spend more time here than in any other part. On average, this comprises about 45% of our slumber time.
Once you’ve been asleep for about 35 to 45 minutes, it’s now time to go deeper. This part is also referred to as “Delta sleep” or “slow-wave sleep” to describe the types of brainwaves that show up on an EEG (electroencephalogram) during a study.
Here’s what to expect during this phase:
- Slower and larger brain waves
- Less likelihood of waking up due to noise and other disturbances
- Disorientation upon waking
In recent years, scientists have combined Stages 3 & 4 and then classified REM as the last Stage.
After you’ve cycled back and forth through all the phases (about 90 minutes), you’ll enter REM. This is the phase where dreaming occurs. As the name “rapid eye movement” suggests, when you’re in this phase your eyes rapidly move underneath your eyelids. Your eyes can go in all directions: back and forth, up and down, and in circles.
Here’s what else happens during REM:
- In addition to dreaming, people prone to sleepwalking and bedwetting may experience episodes during this phase.
- Heart rate and breathing increase and become irregular.
- The first REM stages last about 10 minutes but increase to up to an hour as the night progresses.
What is Light Sleep?
It is characterized by parts one and two in the cycle. During this phase, we are more easily woken up and may not feel rested if we’re roused during this time.
As adults, we spend half or more of our slumber time this way. While this phase is not known for deep restorative rest and repair, it has been shown that we do experience some cell rejuvenation at this point in the cycle. It’s also believed that this is the time where people transfer memories from short-term to long-term storage. Therefore, it’s crucial to pass through this phase for learning.
While the average amount of time in the light phase is approximately 50% of our unconscious hours, people who characterize themselves as “light sleepers” spend significantly more time in Stages one and two.
It’s important to note that some dreaming may occur during this time, though it won’t be complex narratives or stories like the REM cycle. Rather, it’ll be tidbits of seemingly nonsensical information or random images.
What is Deep Sleep?
Stages three and four make up the deep sleep portion of our cycle. It’s in this part of the night that tissue repair and physical growth occur.
If you’re feeling exhausted, it’s most likely due to deep sleep deprivation. The good news is that while you can’t “catch up” on ZZZ’s, you can recover from a deficit.
Here’s how it works: let’s say you typically get about eight hours of shuteye each night. On average, two of those hours are spent in the latter stages. If you pull an all-nighter, you don’t need to get 16 hours of rest the next night to recover. Instead, your body will spend more of the next night in a deep slumber to make up for the time it lost the night prior.
So, for those naysayers who argue that you won’t be able to catch up on rest, you’ll be able to prove them wrong.
Brain Waves During REM and NREM Sleep
It wasn’t that long ago that scientists believed that our brains ceased most activity while we were sleeping. Finally, in the 1950s a new machine was used to measure brainwave activity while we were unconscious. The scientists discovered that our brains exhibit different wave patterns depending on whether we’re awake or asleep, and the waves even change their shape and frequency according to where we are in our cycle.
For example, as we begin to fall asleep, our brains are in Alpha and may briefly enter Theta. This is a small, rolling wave pattern that we’re typically in during visualization exercises and meditation. As we move into deeper rest, the waves shift to larger Theta and Delta patterns.
These are larger and less regularly patterned shapes with brief episodes of “spindles,” or faster, higher frequency waves. These waves are all associated with the NREM phase.
When we’re in REM, brainwave activity becomes faster and less regular. Compared to slow-wave patterns that have .5 to four cycles per second, REM brain activity is observed at 15 to 30 cycles per second.
Sleep Cycle According to Age
As we age, we spend less time in deep sleep and more hours in the early stages. Infants experience almost no time in the lighter levels, while senior citizens spend most of their time there.
The findings are similar for REM as well, Infants are in REM about half of their slumber time, while adults are there about 20% of the time. The older we get, the less REM we’ll experience.
Frequently Asked Questions
What is a sleep pattern?
A sleep pattern refers to the electrical wave impulses measured on an unconscious brain. During the course of waking and sleeping, our brain cycles through waves called beta, alpha, theta, and delta. During the different sleeping phases, scientists can observe the wave patterns using a machine called an EEG or electroencephalogram.
When we’re awake, our brains are in beta. As we fall asleep, we enter into alpha. As we go deeper, well exhibit theta and delta waves with spikes of regular activity called spindles. When we’re dreaming, or in the REM cycle, the patterns become irregular and less predictably formed.
What stage do dreams occur?
While we can experience brief flashes of images during any time, dreaming occurs during REM. In this part of the cycle, our brains become more active, signaling our bodies to increase breathing, heart rate and eye movement. Hence the name, Rapid Eye Movement.
Why is REM sleep important?
It’s in this part of our cycle that our body restores itself. That’s not all though. Scientists also believe that’s an important time for learning and cognitive development. The logic goes that since babies and children spend more time in REM than adults, they’re using this time to experience “waking life.” It helps them process their surroundings and wake up with new knowledge and perspective, even if they’re not aware that they’ve learned anything.
The function of sleeping and what happens in our brains during this nocturnal activity is still somewhat of a mystery. We do know, however, that keeping regular habits and getting the proper amount of rest are both crucial for our physical health and our well-being.
Author: Jill Thompson
I've been self-employed for almost four years and I would not change it for anything! I believe that anyone can achieve their goals with the right attitude and determination.
I'm an avid traveler (25+ countries and counting) that loves to meet new people doing amazing things.
When I'm not researching for the Sleep Advisor, you can find me reading, running, traveling, golfing, or meditating.
I wish you the very best on your journey!