We spend about a third of our lives asleep, which means we are largely unconscious for a significant chunk of our lifetimes. Up until the 1920s, it was generally accepted by the medical community that the human brain practically shut itself off while sleeping.
After the invention of the EEG (short for ‘electroencephalography’) in 1924, things suddenly looked dramatically different. For the first time, it was possible to record electrical activity in our brains. And lo and behold, brain activity was detected during rest.
Thanks to today’s even more advanced technologies, we now know that our brain can actually be more active when we’re asleep than when we’re awake. Each night, we go through 4-6 sleep cycles that each contain four sleep phases or sleep stages.
Curious about the different sleep phases, what they mean and what brain waves have to do with them?
The four phases of sleep
Sleep is basically divided into REM sleep (which stands for ‘rapid eye movement’) and non-REM sleep, the latter of which is comprised of the first three sleep phases and end up accounting for 80-75% of total sleep time.
This results in a total of four sleep phases, which differ from each other by the different brain waves generated during each of these phases, among other things.
- Sleep onset, aka falling asleep or awake (N1)
- Light sleep (N2)
- Deep sleep (N3)
- REM sleep
Passing through all four phases counts as one sleep cycle, a process that takes, on average, about 90 minutes and happens about 4-6 times per night, corresponding to 6-9 hours total of sleep. Slightly waking up between cycles is completely normal, but we usually don’t remember it later. Toward the end of our night’s rest, the phases become shorter and shorter.
It’s during sleep that our bodies regenerate and our memory is consolidated, and each sleep phase contributes to this process in its own way.
Sleep Phase 1: The ‘Falling Asleep’ Phase
Your body relaxes and becomes increasingly sleepy. Your slumber is still fairly light, so waking up is still relatively easy at this stage of the game.
Your brain activity slows down and generates alpha waves at a frequency of 8-13 Hz (short for Hertz, meaning number of wave cycles per second) and theta waves at an even slower frequency of 4-7 Hz. These types of waves can also happen in your brain when you’re awake and very relaxed, like when you’re daydreaming or engaged in creative thinking.
This phase typically lasts 1-7 minutes, before you head into...
Sleep Phase 2: The Light Sleep Phase
Slower theta waves continue to dominate your brain, but now they’re accompanied by spikes in frequency, to 12-14 Hz, that only last about 0.5-1.5 seconds. These brief boosts are called sleep spindles.
One possible explanation as to why this happens is because our consciousness is dipping in and out of consciousness in the light sleep phase. In evolutionary terms, this might have enabled our ancestors to react to potential dangers quickly should they arise in the middle of the night. The sleep spindles could have served as a clock to keep us alert.
Light sleep tends to last 10-25 minutes in the first cycle, but can lengthen with each cycle as the night goes on. Overall, it accounts for about half of your total sleep on any given night.
Sleep Phase 3: The Deep Sleep phase
In this phase, sleep quality is at its highest, as the brain waves slow down to their lowest levels, 1-4 Hrz, which are also called delta waves. This is why deep sleep is also referred to as ‘delta sleep’ or SWS, which stands for ‘slow wave sleep.’
As your mind relaxes, your body slows down too. Muscles become slack, your blood pressure drops significantly and your breathing and heart rates slow down too. That’s why it’s so difficult to wake up from this phase.
It’s believed that this is the phase when the important information processing takes place in your brain, as your body also repairs its tissue and strengthens your immune system.
This phase tends to last 20-40 minutes in the first few sleep cycles and then becomes shorter as the night wears on, shifting this time to REM sleep.
Sleep Phase 4: REM Sleep
REM stands for Rapid Eye Movement — in other words, when your eyes move quickly. During this phase, your breathing speeds up too and becomes irregular, your blood pressure rises and your heart rate is almost as high as it is when you're awake. Your brain takes in more oxygen and your body temperature is at its lowest.
Some of your muscles might be temporarily paralysed— making it hard to respond to your possibly very vivid dreams — but your face and limbs might twitch uncontrollably too.
At the same time, your brain is exceptionally active: The brain waves generated during this phase strongly resemble those that you generate when you’re awake. This is when most of our dreams take place, and lucid dreaming is also more likely during this phase. REM sleep could be important in developing creative problem-solving skills, among other skills.
The first REM phase lasts around 10 minutes and begins around 90 minutes after you fall asleep. The length increases from cycle to cycle, with the last one lasting up to an hour.
In healthy adults, REM sleep accounts for around 20-25% of total time asleep in a given night.
Fun fact: For a long time, there were rumors that the 80s-90s rock band R.E.M. (of ‘Losing My Religion’ fame) named itself after the sleep phase, but when the late Dr. William Dement of Stanford University, known as ‘the father of sleep science,’ asked the band about it, they said nope, that the REM phase wasn’t the origin of their band’s name.
What does sleep paralysis have to do with REM sleep?
Have you ever woken up in the middle of the night and felt immobilised, like you couldn’t move your limbs? This is what’s commonly known as sleep paralysis and some studies have shown that 30% of individuals will experience it at least once in their lifetime. It’s believed to have something to do with a REM phase that’s not fully executed in your body.
It can be extremely uncomfortable — even if we know that it only lasts for a few minutes or seconds, really.
How much deep sleep do I need?
It’s recommended that adults get about 1-2 hours of deep sleep per night. By sleeping 6-9 hours, you allow your body to experience 4-6 cycles in one night and thus 1-2 hours of deep sleep. You can read more about how much sleep do you need in our article here.
How does deep sleep change as we get older?
The older we get, the less deep sleep or delta sleep we’re likely to get, according to this study on adults 26-101 years old.
Interestingly enough, however, another study compared male yoga practitioners to non-yogis ages 20-30 vs. ages 31-55 and found that while deep sleep declined into middle age in the non-yogi group, ‘participants of the middle-aged yoga groups showed no such decline in slow wave sleep states’ — if that’s not a reason to roll out the yoga mat, we don’t know what is!
How does REM sleep differ in children and adults?
While we adults spend 20-25% of our total sleep in the REM phase, in newborns it’s more than more than double, at about 50%.
In addition, newborns sleep up to 18 hours per night (or rather, per day!), which means they spend about up to 9 hours in REM sleep. In addition, some of them go directly into a REM phase when they fall asleep.
As to be expected, the older a baby gets, the more its sleep patterns begin to resemble adult sleep patterns — and we don’t have to wait until puberty for it to happen. Adult-like sleep architecture emerges already by the age of 5.
How to optimise your sleep phases?
Do you often wake up in the morning feeling completely exhausted? It could be because your alarm clock interrupted you during your deep sleep. When this happens, your mental performance can be moderately impaired for 30-60 minutes. (It’s a legitimate excuse if you’re roommates or colleagues are annoyed that you’re lagging behind in the morning...)
Generally, it's best to wake up at the end of a sleep cycle rather than in the middle of one.
Some apps claim to identify which phase of sleep you are in based on your movements (or lack thereof) on your mattress. You select a time range in advance of when you want to be woken up and the app uses your data to select the least disruptive time to sound the alarm. Special wearables also claim to deliver on this promise too.
The most gentle way to start the day is when you don't even need an alarm clock to wake up — or to continue dozing off anyway when it rings until you’re feeling awake. Ideally, you'll wake up on your own between two sleep cycles. Creating your own sleep routine can help you get closer to achieving this.
You can also try to build your entire daily route around your natural sleep rhythm — provided, of course, that you know your own sleep type. For example, if you’re the type that likes to start late or needs a 90-minute afternoon nap, flexible working hours could be a real game changer, allowing your to go ahead and sleep in, or take that afternoon nap.
Skipping that last beer or a glass of wine every now and then could also have a positive effect on your sleep. Alcohol is suspected to throw off your natural sleeping rhythm.
There’s an eternal dispute about which diet is best for good sleep. This small study from 2008 concluded that slow wave or deep sleep levels were increased in individuals who followed a low carb diet.
Another slightly larger study showed that moderate to strenuous exercise in the evening was also associated with better sleep patterns.Considering how closely our brain activity is related to our sleep, our mental state before bedtime is likely to play a role in our sleep quality too. While more research must be done in this area, some studies already suggest that CBD could improve sleep — and many people already report positive results.