The science of sleep – the stages of sleep and the circadian rhythm

Getting enough sleep is crucial for your wellbeing, as well as your physical and mental performance. Despite its importance, only two-thirds of adults get the recommended 7-9 hours of shut-eye per night. The remaining third get by on six hours or less.

Everyone needs sleep, but a lot of the aspects of sleep are a mystery – even to the scientific community. That said, there are lots of things we do know about sleep, two of which are the stages of sleep and the circadian rhythm.

Understanding these two sleep factors will help explain not only the importance of getting enough sleep but why sleep quality is as vital as sleep duration.

The stages of sleep

A lot is going on while you sleep. Sleep affects every organ and system in your body, from your brain to your heart to your lungs, and right down to your cells. There are four stages of sleep, and you cycle through each one several times per night. Each stage has a different effect on your body and your brain.

The four stages are: 

Stage one – during this stage, your body starts to transition from wakefulness to sleep. This stage lasts no more than a matter of minutes, during which your heart rate, breathing rate, and eye movements start to slow. 

Your muscles also relax, which is often accompanied by occasional twitching. This is why your head may nod as you start to doze. This stage of sleep has no real therapeutic effect but is thought to be when short term memories and skills are uploaded into your subconscious and turned into long-term memories. You are easily woken during stage one sleep.

Stage two – this stage of the sleep cycle lasts 10 to 25 minutes and makes up around 45% of your total night’s sleep. This stage is usually thought of as “light sleep.” Stage two sleep is restorative but is not deep enough to make significant inroads into recovery and energy restoration. Because this is not a deep sleep state, you are easily woken at this time.

Stage three – making up around 25% of your night’s sleep, stage three sleep comes in bursts of around 20 to 40 minutes. During this stage, blood flow to muscles increases, blood pressure decreases, and human growth hormone and testosterone production increases. Your energy levels are restored, and your body produces the anti-inflammatory substance prolactin, which helps to repair your joints. 

Stage four – this final sleep stage is also known as rapid eye movement or REM sleep. It makes up about 25% of your total night’s sleep. REM sleep typically comes in bouts of 10 to 60 minutes, and usually occurs 80 to 90 minutes after first falling asleep.

During REM sleep, your muscles are immobile, but your brain becomes increasingly active. This is when you dream. The reason we dream is unclear, but some experts believe it’s part of the brain’s rebooting process, during which it clears out information that it no longer needs.

REM sleep is also when your body gets busy repairing minor muscle damage, restoring depleted energy, and infusing your tissues with lots of freshly oxygenated blood. This stage of sleep also boosts daytime mental performance.

Each sleep cycle lasts about 90 minutes and most people complete 4-5 cycles per night. Waking during the night can break the flow of the stages of sleep and leave you feeling tired. Because of this, sleep quality is deemed to be as important as sleep duration.

For example, if you keep waking up every 30-45 minutes, you’ll never enter the latter stages of sleep, and you won’t experience the effects those stages provide. That’s why you may still feel tired after what seemed like a full night’s sleep; you didn’t spend enough time in all four stages of sleep.

Short naps are also not as replenishing as a full night’s sleep, even is you accumulate the same sleep duration. It seems that your body needs to sleep in cycles of around 90 minutes to acquire any real benefits from sleep.

Takeaway: While you have no control over the stages of sleep, it’s important to understand that sleep quality is as essential as sleep duration. Light sleepers, who do not spend much time in stages three and four, won’t be as well rested as those who complete several cycles of all four stages of sleep per night. Being comfortable in bed, darkness, an ambient temperature, and silence make it easier to sleep more deeply, and that means that you’ll feel more rested after a night of sleep.

The circadian rhythm 

Your body has a biological clock called the circadian rhythm. This clock regulates your sleep cycle and determines when it’s time to sleep and when it’s time to be awake. The circadian rhythm is controlled mainly by exposure to light and regulated by hormones such as melatonin and serotonin.

This should mean you feel wide awake during the daylight, and naturally want to sleep at night. If you didn’t know what time it was and had no artificial lights, you’d naturally want to sleep when it started to get dark and wake up as the sun rises. 

Unfortunately, artificial light and exposure to things like computer and smartphone screens can lead to circadian rhythm confusion, extending the hours of wakefulness long beyond the time you should be sleeping. Also, you can ignore your natural desire to sleep and chose to stay awake for longer. This further disrupts your circadian rhythm.

While occasional sleep disruption is entirely normal, many people go through life ignoring their circadian rhythm and suffering from insomnia. They want and need to sleep, but find that they can’t, or that they wake up during the night and are unable to doze back off.

Takeaway: Before the advent of electric lights, sleep and wakefulness were primarily governed by sunset and sunrise. Exposure to bright light primes your body for wakefulness, even if it’s time to go to bed. Because of this, a lot of people struggle to get enough sleep. They might feel tired, but light exposure makes sleep much harder to come by. 


We don’t understand everything there is to know about sleep, but what we do know suggests that sleep quality is just as important as sleep duration. You need to complete all four stages of sleep several times to wake feeling rested and energized. Interrupted sleep can be as problematic as not sleeping enough.

Because your natural sleep cycle is influenced by exposure to light, modern inventions such as electric lights and computer screens can make getting enough sleep more complicated than it was for our less technologically advanced ancestors. In many cases, disrupted sleep can be prevented simply by limiting light exposure late into the night.

Sleep, function, and health are inextricably linked, and that means it is essential that you try to get the recommended 7-9 hours of uninterrupted sleep per night. Understanding and respecting the science of sleep can help.

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