At this point of your life, you can firmly say you have a pretty vast experience when it comes to sleeping. You spend (or at least should) ⅓ of your time on earth doing it. There is probably no other single activity that you dedicated so much time to. But how much you know about the things that happen to your body and mind when you’re sleeping?
Ingredients of a perfect night
It might be surprising but our knowledge of the existence of the sleep cycle is fairly new. In fact, we have been aware of it for much less than 100 years. Sleep stages were first identified in 1937, yet it took another 20 years to determine the actual differences between them. Those differences are easiest to explain with the use of a hypnogram.
But what exactly is a hypnogram? In short, it is a graph that represents the depth of sleep over time. If you ever heard the term sleep architecture and wondered where do the name come from – just look at the graph. The highest peaks resembling the skyscrapers are a representation of the awake stage. It might be surprising, but each of us experiences even up to 10 micro-awakenings in the course of one night – and we usually don’t remember any of them. The lower levels of the scrapers represent the REM stage. It’s the one that is closest to being awake in terms of consciousness. The even lower ones are NREM 1, NREM 2 and NREM 3 respectively. Let’s examine them first.
The first stage that you usually enter when you got to sleep is NREM1. It is a transition between being awake and asleep. It passes relatively quickly, as it lasts only around 5-10 minutes which accounts for about 5% of the cycle. Your body temperature lowers down and your muscles begin to relax. While you might not be totally aware of your surroundings in this stage, it is very easy to wake you up, by, for example, saying your name or opening the bedroom’s door. Some people might experience NREM 1 more as a form of extreme drowsiness rather than actual sleep.
In this stage, you can also experience a unique phenomenon called hypnic myoclonia. It is a very sudden but, in most cases, completely harmless twitch often associated with the feeling of falling down followed by a sudden awakening. About 70% of people experience it at some point in their lives, but don’t worry – it’s completely normal. Other phenomena you can encounter during NREM 1 are hypnagogic hallucinations. They are short experiences of hearing, feeling or seeing something in your close surroundings. While it can be a little frightening, those hallucinations are also completely harmless.
The second stage, NREM 2, is the one in which you spend the most time during the night. When you enter it, your sleep becomes deeper and you become considerably harder to wake up. The awareness of your surroundings fades away completely.
In this stage, your brain experiences two types of short bursts of activity. The first type called k-complex is responsible for keeping you asleep. If you are sleeping in a noisy environment or share a bed with a restless partner, this mechanism makes you ignore that stimuli and carry on with your rest. The second type of brain activity is called sleep spindle. It is thought to be responsible for information processing and memory consolidation – they influence the process of transforming new information into actual memories.
Because of those mechanisms, this stage might be of particular importance if you are trying to learn and remember something from the previous day.
NREM 3, often referred to as deep sleep, is the stage that is the furthest from the awake stage. After entering it, you are basically cut off from most of the stimuli coming from your surrounding. Waking up becomes most difficult in this stage, and if you actually do it there is a big chance that you will feel groggy and quite tired.
In the NREM3 stage, your body and mind enter the most relaxed state. Your brain activity is the slowest and your muscles relax. Your breath and heart rate reach their lowest point of the day. The processes that occur in this stage are generally responsible for repairing and healing the body – they aid tissue growth & muscle repair and defend infections. The curative effects of this stage are especially visible when you consider how much you want to sleep when you are ill – in many cases, good sleep works as the best medicine.
The amount of time that is spent in this stage highly varies depending on your age. Newborns experience a lot of NREM3 which is thought to be crucial for them to grow and develop normally. Sadly, as we get older it’s duration shortens gradually and in our latest years, we barely experience any deep sleep.
The REM stage is a unique stage that you usually tend to enter as the last one in a single sleep cycle. In this stage, your brain becomes active again and your eyes start to move. In fact, this stage takes its name from those rapid eye movements.
The other characteristic of REM is that after entering it your body becomes paralyzed. All of the muscles except the ones responsible for eyes and breathing become completely still. While it might sound a bit scary, it is thought to be a safety mechanism of some sort. The muscle atonia prevents us from hurting ourselves by physically reacting to what in our dreams – and most of our dreams do occur at this stage.
For a long time, we have thought that REM is the only stage in which we experience any dreams at all. It’s a relatively new revelation that dreams actually can occur in the other three stages as well. Still, there is a certain difference between the ones that we experience in REM and NREMs. On the beginning of the sleep cycle, they usually tend to be much more realistic and widely based on memories. Those dreams don’t usually get very crazy – they might be just a reminder of what happened the previous day. But as the cycle progress and as we enter the REM stage, they become longer and much more surreal and vivid. We also tend to remember them much better than the NREM ones. That is why we usually get the impression that we dream the most in the morning as then we experience the most REM sleep.
The key is in the dose
All four described stages come together to create one full sleep cycle. In ideal conditions, one sleep cycle should last for more or less 90 minutes and be repeated about 5 times over full sleep period. Generally, the consecutive cycles differ from each other a bit. Most deep sleep (NREM 3) happens at the beginning of the night. Contrarily, REM sleep increases in length through the cycles and tends to be the longest in the last one. That is why sleeping for just one or two cycles is definitely not enough for you.
It’s also worth to mention, that the duration of each stage and cycle can vary heavily in two consecutive nights too. The timespan of stages is based on your current needs which might be a bit different from day to day. Each stage is responsible for something else, either in your body or mind, but only together they work as a perfect medicine that you need. And given that you don’t suffer from any sleep related issues your body will know what is the right dosage – which in the case of most adults should be around 7-9 hours.
- Epstein, Lawrence J. (2010). Improving sleep: A guide to a good night’s rest. Boston, MA: Harvard Medical School.
- Walker, M. P., & Van der Helm, E. (2009). Overnight therapy? The role of sleep in emotional brain processing. Psychological Bulletin, 135(5), 731-748.
- Wilson, S., & Nutt, D. (1999). Treatment of Sleep Disorders in Adults. Advances in Psychiatric Treatment, 5(1)
- Marano, H. (2003, November 1). How to Get Great Sleep. Retrieved from https://www.psychologytoday.com/articles/200311/how-get-great-sleep