Sleep has always been an intriguing phenomenon for us and a mystery to say the least. When reading more about the process of sleep, you will often come across the term “sleep cycle”. This term refers to the progression from stage one to stage R, or REM sleep, that occurs during the night.
You see, sleep has been studied upon extensively over the past few centuries but there’s still a chock full of information that we’re not completely sure about. What we do know, however, is how sleep proceeds throughout the night, otherwise known as the different stages of sleep.
What Are the Stages Of Sleep?
Sleep comes in a variety of stages (five, to be exact!). There are some more restless hours in a night’s sleep, and some filled with a load of activity, dreams and movements. Broadly speaking, sleep can either be non-rapid eye movement sleep (NREM) or rapid eye movement sleep (REM).
The NREM sleep is further divided into four stages (differentiated on the basis of brain activity and EEG readings.) These stages are numbered one through four and all of them have some common characteristics that I’ll touch on in a little bit.
The REM stage is on the other end of the spectrum and this is when your brain is most active.
Before sleep actually sets in, there is a short period if latency (of around 5-15 minutes). Some people will have a longer period of latency, while others will fall asleep instantly.
So you’ve fallen asleep – congratulations! Now what’s going on?
Well, the activity in your brain is starting to slow down. These brain waves are picked up by an EEG and are described by health professionals as the theta waves. Theta waves are slow but not slow enough.
Stage 1 sleep, like all other stages of NREM sleep, causes several different effects on your body. These include tenser muscles, difficulty in arousing, slow or absent eye movements and absent mental activity. However the body at this stage is movable.
Many sleep experts will call the NREM sleep an idling brain in a moving body – quite apt, if you ask me!
Stage 1 is the shortest of all sleep stages for most adults.
Stage 2 comes next in line and the brain waves begin to slow down even more. This is when you’ll see the characteristic sleep spindles and spikes on an EEG reading. Stage 2 is slower, deeper than stage 1 and has more prominent characteristics of a NREM sleep.
Again, the person becomes difficult to wake up at this point, has no recollection of any dreaming or any mental activity, has a high muscle tone but the body moves on its own.
It’s usually in the stage 2 (or the REM) sleep that a person wakes out of. An average adult will spend most of his or her night in the stage 2 of the sleep.
STAGE 3 & 4
Often combined as one, stages 3 and 4 of the sleep are also called as the ‘delta sleep’ or the slow-wave sleep. This is when the deepest, slowest brain waves called delta waves begin to show on an EEG. The first bout of slow-wave sleep occurs within the first hour of sleeping and this is the more restorative part of the sleep where memory consolidation occurs.
On average a person will have around 4-5 slow-wave cycles. It’s extremely hard to wake someone up once they have entered the slow-wave cycle of their sleep but the body is movable. Delta sleep lasts only for a few minutes before the next stage takes over.
If you’ve ever woken up tired and restless after a short sleep, it’s because you haven’t had enough of the slow-wave sleep. Interestingly enough the duration of your slow-wave sleep is around 65% inherited – how cool is that?
The most ‘popular’ stage of the sleep cycle is the REM sleep. You’ve probably already heard this word thrown around everywhere in shows, TV and books. By now REM has become a pretty generic term but do you really know what it means?
The REM stage, or the rapid eye movement stage, is when the brain is most awake (as picked up on an EEG). Someone in REM sleep will have brain activity almost similar to a person that is fully awake and conscious. But (here’s the catch) the body is completely paralyzed at this stage.
REM is also characterized by rapid saccadic eye movements in a motionless body. You are more likely to get sexually aroused in the REM stage as well.
If you’ve ever woken up from a nonsensical but highly elaborate dream, there’s a good chance you’ve just had your REM stage of sleep. Vivid dreaming with an equally vivid recollection of it is characteristic of REM sleep. You’re also easier to wake up out of this stage.
An average adult will have his first REM in around 90 minutes with a total of 5-6 bouts of REM sleep in an 8 hour period. As the night progresses the slow-wave cycles get less frequent while the REM stages get more and more frequent. In fact most of the REM that we have is concentrated in the later part of night.
What Is A Sleep Cycle?
One sleep cycle is defined by the total period of NREM to REM sleep before the next NREM stage appears. Typically a healthy adult should have around 4-5 sleep cycles in an 8-hour period.
The duration of the sleep cycle varies as the night progresses. Usually the first sleep cycle is around 90 minutes long but the subsequent cycles grow longer (around 120 minutes). The ratio of REM to NREM also increases with the number of sleep cycles.
Duration of Deep Sleep
The first slow-wave cycle will usually appear within the first hour and the total duration of your deep sleep has a genetic factor associated with it. So if you’re a light sleeper or someone that has trouble having a good night’s sleep, maybe your genes are to blame for it.
Deep sleep or slow wave cycles get less frequent as the night progresses and by around 5 hours they completely stop showing up. Perhaps this is just how our bodies are wired to stop us from sleeping too long.
Without a doubt deep sleep is the most restful and restorative sleep in your cycle. You need this stage of sleep to get you going in the following day. There’s also a good amount of evidence that memories are consolidated during this part of the sleep.
When does REM Sleep Occur?
There’s a period called the REM latency between the time we fall asleep until our first REM stage. This is usually about 90 minutes or 1.5 hours. REM becomes more and more frequent with the hour of sleep but the actual duration of the REM sleep is only about 30 minutes or so.
REM sleep in a healthy adult usually occurs in 90 minutes intervals in the first part of the night but this interval gets shorter with time.
When Do Dreams Occur?
Many people think that dreams occur only in the REM stage but that’s actually not true. Believe it or not, dreams occur throughout the night regardless of the stage of the sleep.
It’s remembering those dreams that make the difference in the stages.
When you’re in your NREM stages of sleep you’re more likely to have almost no recollection of your dream. If you do wake up out of a NREM stage, you might momentarily remember the dream but you’ll forget it in a few minutes.
REM sleep is different. This is when you’ll actually remember your dreams, in most cases clear as a day! Nightmares usually occur in this stage as well. Because REM sleep is concentrated in the later hours of the night, you’re more likely to wake up from a vivid dream you’ve seen in your REM stage.
What Affects Sleep Stage Distribution?
This is a no-brainer: we all know that certain factors can make or break a good night’s sleep. While some of these are in our control, other factors are not. Sleep distribution is affected by:
- Age: Our pattern of sleep changes throughout life. Infants are deep sleepers and have long sleep cycles. They also have low REM stage sleep. As we grow older we get less and less deep sleep. That’s why most elders are light sleepers and frequently wake up throughout the night.
- Deranged Circadian Rhythm: Now this is something that can totally be wired for a good sleep distribution. Our bodies have an inbuilt circadian rhythm that is dominated by the hormone melatonin. This hormone gets released in a cyclical fashion and how you sleep in the previous nights can change the cycle. If you haven’t had a good sleep in the previous night, your body will make up for it and increase the NREM stage of sleep the next time you hit the Z’s.
- Temperature: Believe it or not but the temperature in your room can also have a major effect on your sleeping pattern. It has been observed that we tend to sleep better in slightly cooler environments. Anything too hot or too cold can affect our sleep.
- Medication: There are certain medications and drugs that can change how you sleep.
- Disorders of Sleep: Sleep disorders can take a toll on your sleep cycle and affect your sleep clinically. Disorders such as sleep apnea and sciatica are medical conditions that need to be addressed by a professional.
Nobody fully understands the importance of sleep until they’re sleep deprived. If you’ve been tired lately and haven’t had a good sleep in a while, it’s a good idea to evaluate how your sleep cycle is working.
Are you getting enough deep sleep? Is there something affecting your sleep distribution? Or are you suffering from some sleep pathology? Whatever it is, your doctor will be able to help you figure out and find the solution.
About The Author:
Chris is the Founder & Publisher at SleepStandards.com. He aims to inspire better sleep by providing research-based sleep health advice, actionable sleep tips, and unbiased sleep product reviews. Find out more about his work at SleepStandards.com.