Sleep is an important part of your life. And it’s not just a part of your life; it’s an activity you’ll spend about a third of your life doing, or at least trying to do. If fact, you probably hit your 10,000 hours of sleep by the time you were a three year old.
If you’re not a sleep genius by this time, it’s never too late to start. Perhaps by better understanding what makes for quality sleep, it might become just a bit easier to get it right.
Stick with us as we walk through the different parts of a sleep cycle.
Each night you go through four to five sleep cycles — depending on your personal sleep needs.
Each of these cycles is comprised of various stages of sleep. Each of these stages is essential to maintaining the health of your body and your brain.
In fact, a good night’s sleep is just as important as food and water to your overall health. Without sleep, you wouldn’t be able to form new memories or concentrate throughout your day.
Let’s take a look at the components of these sleep cycles — what makes for healthy progress through those cycles and how they affect your everyday life.
What is a sleep cycle?
Even though you need sleep to rest your body, you remain pretty active while you sleep. Only 50 years ago, doctors thought that your body shutdown while you slept. But now we know that this isn’t the case.
Your brain can be more active while you’re asleep than it is during the day. There are many stages of sleep that your body goes through during the night.
Sleep cycles are divided into two categories: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (Non-REM or NREM) cycles. There are also different levels of NREM. When you move through all of the stages of NREM and one stage of REM, that is a normal sleep cycle.
And you go through four to five of them throughout the night — or at least that’s what should be happening.
NREM vs. REM Sleep
NREM sleep is composed of three different stages: N1, N2, and N3. There used to be a fourth NREM stage called N4, but in recent years researchers have found that there are no discernable differences between N3 and N4. So now only three stages of NREM sleep are recognized. NREM are deeper sleep cycles and make up 75% of the sleep you get each night.
REM sleep is a cycle that starts about 90 minutes after you fall asleep. This is a lighter sleep cycle and occurs after you have already gone through three stages of sleep. This phase only lasts about 10 minutes, but it gets longer throughout the night. You spend about a quarter of your night in REM sleep.
An in-depth look at the sleep cycle stages
There are four stages of sleep. Three of these stages are non-REM and one of the stages is REM. During a night of sleep, your body cycles through these stages in a specific sequence. Throughout the night, the amount of time that you spend in REM gets longer and longer. On the other hand, we get are deepest NREM sleep earlier in the night, and then these stages get progressively shorter into the early morning when REM sleep dominates.
Stage one: N1
When you start to get drowsy, you are drifting into the first stage of sleep. This is non-REM sleep and is the transition from someone being awake to going to sleep. N1 is a light stage of sleep and lasts between five and 10 minutes. During this stage, your heart-rate and breathing begin to slow down. Your muscles start to relax, eye movements slow down, body temperature drops, and brain waves slow down.
In the first stage of sleep it is easy for your to be woken up. In fact, if you are woken up from this stage of sleep, you might not even know that you had fallen asleep. When you take a nap, N1 is the first stage of sleep that you enter.
I’m sure all of us are familiar with drifting off and then suddenly jerking awake. This happens during the N1 sleep stage, and are known as “hypnic jerks”. This muscle spasm can also occur alongside a falling sensation. Sometimes a hypnic jerk will wake you up, and other times you will sleep through it.
You spend the least amount of time in this stage of sleep. Overall, you will only spend about 5% of your sleep time in N1 sleep.
Stage two: N2
Once the N1 stage of sleep ends, you enter your second stage of NREM sleep. You spend between 10 to 25 minutes in N2 sleep. This is also considered light sleep. During N2 your heart rate slows down further, eye movement stops, your brain waves get slower, and your muscles become even more relaxed.
You spend most of your sleep in the N2 stage. Remember, you cycle in and out of all of these sleep stages in a specific sequence. You spend 55% of your total sleep time in the N2 stage.
Stage three: N3
This is the final stage of NREM sleep. This sleep stage can be referred to as slow-wave, deep, or delta sleep. When you are in this stage, your brain waves are known as delta waves, which refers to the slow, predictable frequency that these brain waves oscillate at. This is your brain functioning at its lowest activity.
Slow-wave sleep is necessary for you to feel rested and rejuvenated. You spend more time in the N3 sleep stage in the first part of the night. Experts are unsure why this happens, but it’s just the sleep routine that our bodies go through.
The N3 sleep stage lasts between 20 and 40 minutes. During this phase of sleep, it is the most difficult to wake someone up. This is due to the fact that your brain is not responsive to external stimuli. Your brain is in such a deep sleep state that someone touching you wouldn’t even interrupt this stage of sleep. If you are woken up during N3 sleep, you would feel incredibly groggy.
In N3 sleep, your breathing and heart rate are at their lowest levels. Your blood pressure drops, and your body temperature gets even lower. There is still no eye movement and your muscle tone is low. Even though your blood pressure falls, it isn’t dangerous. This is just a normal part of the sleep process.
Although this is the deepest stage of sleep, this is when sleepwalking occurs and when people experience night terrors. These sleep-related conditions will not occur in any other sleep stage. Overall, you will spend 15% of your night in N3 sleep.
You go into REM sleep 90 minutes after you go to sleep because first you have to cycle through all of the other sleep stages. Your first REM sleep cycle lasts only 10 minutes, but the length of subsequent REM sleep stages increase throughout the night.
During REM sleep, your eyes move quickly back and forth beneath your eyelids. This doesn’t constantly occur throughout the sleep stage, but it does happen. Many scientists link this eye movement with dreaming. While you are in REM you will have the most vivid dreams. If woken up from REM sleep, you might remember parts of these dreams.
In REM sleep, your heart rate will increase along with your blood pressure. However, your body temperature is the lowest that it is throughout the night. Your arm and leg muscles are completely relaxed and do not move. Experts believe that this immobility generated in the brain occurs to keep us from acting out our dreams physically. However, in one sleep disorder called REM sleep behavior disorder, muscle paralysis is disrupted in this stage and individuals can physically enact their dreams.
Your brain can be more active in a REM sleep cycle than it is when you are awake. During REM, your breathing becomes faster and shallower. Sleep specialists believe that REM is when your brain processes all of the information from your day to create, interpret and manage memories and emotions. You spend between 20% and 25% of your night in REM sleep.
How long is one sleep cycle?
Your first sleep cycle takes about 90 minutes. This is how long it takes for your body to go into REM sleep. After your first cycle, your body will average 100 to 120 minutes per sleep cycle. During the night, you will go through 4 or 5 sleep cycles. The sleep cycle time can vary slightly depending on the person, but these are the average sleep cycle times.
How long is each sleep stage?
Depending on where you are in the trajectory of your night’s sleep, the stage can vary in length. Each person has a different sleep schedule that works for their body. So, how long is one sleep stage? The shortest sleep cycle is N1 when you are trying to fall asleep. N3 is the longest sleep cycle, where you will spend 30 to 40 minutes in this sleep state.
Sleep staging in a normal sleep cycle
As you know, you drift through four stages of sleep in a specific sequence. The most accurate way to know how you move through these stages is to participate in a sleep study at a sleep clinic. Sleep studies monitor your brain activity while you sleep, to determine how long you are spending in each specific sleep stage. The normal breakdown for the stages of sleep cycles:
- N1: 10 minutes
- N2: 10-25 minutes
- N3: 20-40 minutes
- REM: 10 minutes (but this cycle with get longer throughout the night)
It’s worth noting that most consumer sleep trackers use only accelerometers to measure sleep and make estimations based on movement to predict REM and slow-wave sleep. We go into a bit more detail in this article on sleep trackers.
How do I know if I am getting the right amount of sleep?
We can’t connect ourselves to machines at home to monitor our brain activity. So, how do we know that our sleep cycle time is healthy? When you wake up and feel refreshed, you are getting a healthy amount of sleep. It’s not always about quantity. In fact, it’s not uncommon to wake up after a full eight hours of sleep still feeling tired.
Sleep experts believe that there is a mental cost to processing memories in our sleep. The greater the processing the more mentally taxing. Dr. Bob Stickgold, Professor of Psychiatry and Director of the Center for Sleep and Cognition at Harvard Medical School, expanded on this crucial memory processing function that occurs in REM sleep:
“It's called memory tagging. And it seems to be a way that the brain can put a yellow sticky on a memory. We think it puts a physical, chemical tag on to some of the connections that make up a memory, [to identify] that is in need of further processing. It's something for which the brain has calculated it doesn't have as much information as it wants.”
However, this system can get overloaded with highly emotional content that needs further processing, which can result in non-restorative sleep. Dr. Stickgold continues, “As we overload, as we have more and more things that are both emotional and not completely processed that fall into that category of what sleep will tag and try to deal with one way or another.
"As we get more and more things tagged that way, we reach the limit of the system's capacity. When that happens and you get through the end of the night, and you can feel that there's still unprocessed material that your brain had wanted to process, you sort of wake up and you feel exhausted, you feel like you were working all night.”
For most of us though, improving our sleep quantity is the best place to start. When you are missing out on your sleep cycles, that’s when you really start to feel the effects of sleep deprivation.
What is a sleep-wake cycle?
Your circadian rhythm regulates what is known as your sleep-wake cycle. There are times of the day that your body naturally wants to be asleep and times your body wants to be awake. For example, if you work during the night, it can be difficult to train your body to sleep during the day. Your body naturally wants to be asleep at night. For most people, the body wants to sleep seven to nine hours and stay awake for 15-17 hours.
Your body has many hormones and chemicals that help it function throughout the day. These chemicals and hormones can make you feel awake or sleepy, depending on their function.
One of these chemicals, called adenosine, builds in your body throughout the day from the moment you wake up in the morning. As the day turns to evening and then to night the adenosine accumulation makes you feel sleepy. At the end of the day your adenosine levels are at an all-day high, which is why you want to go to sleep. Caffeine actually blocks the action of adenosine, which is why coffee can help keep you awake.
Melatonin is another important hormone that your brain produces to help regulate your sleep-wake cycle. In the early evening, your melatonin levels rise in your body as light exposure decreases. They continue to rise throughout the night, then they taper off in the early morning hours. If there are too many bright lights in your bedroom, your body won’t produce the proper amount of melatonin.
You can even use your relationship with light to help trick your body. If you have to sleep during the day, using blackout curtains can be helpful. You can also turn a light on as soon as your alarm goes off, to encourage your body to wake up.
How to reset your sleep cycle
Having a proper sleep schedule affects every facet of your life. If you find that you are waking up exhausted, you aren’t getting the right amount of sleep. It’s quite possible, for instance, that you are getting too few complete sleep cycles through the stages we discussed earlier.
Thankfully, there are things you can do to reset your sleep cycle:
Turn on the lights first thing in the morning
Sleep, and more generally, our bodies, are greatly influenced by our environment. If there are lights on around us, our brain thinks that it is daytime. So, if you are struggling to get up, try turning on lights as soon as the alarm goes off. Additionally, light therapy boxes are a good option because they mimic bright outdoor light.
You should also be sleeping in a pitch-black room with minimal distractions. Most experts recommend removing all unnecessary screens and lights from your bedroom.
Try a fast
Studies have shown that a fast for 12-26 hours can help you reboot your sleep cycles. Our bodies operate on a routine based on cues called ‘zeitgebers’. Meals and mealtimes act as some of these zeitgebers.
By fasting, you can trick your body into thinking that it is time to sleep. Then when you eat, your body will think that it is morning. Experts recommend eating an early dinner and not eating again until breakfast. Once your sleep schedule is fixed, you can eat normally.
Consistency is key
Your sleep routine is one of the most vital parts of getting healthy sleep. Follow the same routine each time you go to bed to get the best sleep possible.
If you’re interested in getting started with a sleep routine, check our three-step sleep routine article.
How can I optimize my sleep based off sleep cycles?
There are ways that you can get better quality sleep. Most sleep cycles are affected by the same things, so here are tips to get four to five cycles a night:
- Don’t have caffeine close to your bedtime.
- Kick the antidepressants.
- Avoid nicotine.
- Avoid alcohol.
- Set your alarm to go off after your last sleep cycle. If the average sleep cycle lasts 90 minutes, set your alarm to wake up after your last cycle ends. So, you’d set an alarm for 6-7.5 hours after you go to bed. Alternatively, you could try apps like Sleep Cycle which feature a smart alarm clock that attempts to wake you up in your lightest phase of sleep by analyzing your movements.
- Go to bed at the same time every night
- Wake up at the same time every day. Even on weekends.
What happens if I wake up mid-cycle?
Waking up in the middle of a sleep cycle can leave you feeling disoriented. It can also be frustrating because you want to finish getting your sleep. Technically, there are some health risks associated with interrupted sleep, but waking up occasionally won’t hurt you.
If you wake up in the middle of the night and can’t fall back to sleep, get out of bed. Don’t turn on bright lights and try a relaxing activity. You could read a book until you are tired, then try to go back to sleep.
Sleep cycles are an important part of your mental and physical health.
Without a proper sleep schedule, your brain and body will suffer. You could find yourself struggling to make it through your days.
Now that you know a bit more about sleep cycles and their components, you’re on the right track to quality sleep. Check out our article on sleep hygiene to put your best foot forward in building out habits that will encourage better quality and quantity sleep.
Delta wave: Neural oscillations in the 0.5Hz-4Hz range. They are associated with stages 3 and 4 of NREM (slow-wave sleep).
Circadian rhythm: Our internal biological clock that plays an essential role in regulating and maintaining our sleep-wake times, eating cycles, and hormonal cycles.
Sleep hygiene: The different practices, habits, and behaviors that one should adopt in order to both get better sleep at night and be more alert during the day.