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The mystery of sleep

We have managed to measure the effect sleep has on the brains of people, mammals and birds by measuring the brain activity directly.

The mystery of sleep


Sleep is a critical part of our daily lives. Out of the fixed 24 hours in a day, about a third of it is taken up by sleep. It's not something we can live without, and even short periods of sleep deprivation can have a noticeable impact on us. We tire, our brains do not work as well, and it has a negative impact on our general health. Sleep is so important that all animals need sleep. Mammals, birds, fish, even insects sleep. During this time, they cannot eat, drink, reproduce, they are defenceless to predators and cannot fend for themselves unless woken up. Prolonged sleep deprivation has even been shown to cause death in rats, dogs, fruit flies, even in people. To risk being in such a vulnerable state, and to cause so much damage without it, sleep must have an essential function, one that cannot be skipped. So what is it?

We cannot say for certain. But we are getting closer to finding out.



We have managed to measure the effect sleep has on the brains of people, mammals and birds by measuring the brain activity directly. We can see changes in activity patterns, different voltage spikes as the neurones in our brains change how they fire. But at its core, sleep is defined by the behaviour changes. It is a dormant behaviour state, one in which the animal is found immobile, less responsive to the stimuli of the outside world, with reduced sensory intake. Unlike hibernation or a coma, it is possible to rapidly reverse this and enter an alert state. When an alarm goes off or failing that someone pours water on your face in the morning.

We can observe sleep states in most animals, our pets, in birds that roost at night. But even insects have been shown to have inactive periods. The common cockroach was the first to display this inactive state. Many experiments on core biological functions are performed on fruit-flies. These insects have a brain with just 100,000 neurones (compared to the human’s 100 billion) but even these simplistic brains need their sleep. If we go even simpler, the nematode worm C elegans has just 302 neurones in its entire body, all of which have been mapped to completion. It takes just 3 days for it to transition from an egg to an adult. And even it needs sleep.  If sleep can be traced back to such simplistic, basic life forms, it must be essential to the core health of a living being.



Not all sleep is the same type of sleep. At night our brains cycle through many different stages of sleep, each with its own characteristics. They are split into two basic types. Rapid eye movement (REM) sleep and non-REM sleep.

Stage 1 non-REM is when we transition from wakefulness to sleep. In this short period our heart rate, breathing and eye movements slow, and muscles relax. Our brain waves slow down from their normal wake patterns.

Stage 2 non-REM is light sleep, before transitioning to deep sleep. As our heart rate and breathing slowly, even more, our body temperature also drops, and eye movements stop altogether. Our brain activity is slow, with occasional bursts of activity. This is the stage in which we spend the most amount of time

Stage 3 non-REM is deep sleep. This is what you need to feel refreshed in the morning. It occurs the longest during the first half of the night. At this point, it can be difficult to wake a person.

REM sleep occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side underneath your eyelids, and your brain becomes quite active, similar to that seen when awake. Breathing is faster and irregular, as is your heart rate. Its during this time people dream. Your muscles become temporarily paralysed to prevent them from acting out the dream. The functions of dreams is not known, and studying it is so complex we cannot cover it in this blog.



This is the most important question and one we struggle to answer. Studying sleep is difficult. When a person is sleeping, they naturally can’t respond to questions. Animals are even worse at answering these questions.

Trying to measure brain activity non-invasively is hard. Wearing electrodes on your head can show electrical activity, but it is very difficult to know which parts of the brain the activity corresponds to. Using an MRI machine, it is possible to see changes in blood oxygen levels in different parts of the brain. This can show which parts are actively consuming oxygen, and therefore very active, but it isn’t able to tell us when these activities are occurring. Finally sleep deprivation studies are unethical, especially due to the harm they can cause. So that’s out the picture.

From what we have managed to piece together there are two leading theories on the function of sleep.

  1. It allows repair and clearance needed to correct and prevent neuronal damage
  2. It allows the brain to reorganise neurones and engrain our learning.

Both of these activities require a time when the brain is not highly active, and sleep provides the perfect time for this to happen.

When we are awake the neurones in our brain are highly active. This can lead to a build-up of metabolites that can damage the neurones. An excessive build-up of these ages than lead to protein damage, the build-up of dangerous toxins and poorer neuronal health. As we sleep, the metabolic requirements for the brain drop, providing the optimal time to clear these toxins. When we sleep the fluid that surrounds our brain increases in volume and circulation, clearing out these damaging toxins.

Secondly, when we are awake, we are laying down new memories, experiences and learning points. These memories exist as junctions between neurones. The more memory or skill is used, the stronger that junction becomes. Our brain can form erroneous junctions as well, for tasks that are not used often or not useful at all. If this “noise” is kept, it impairs the functioning of our brain and the maintenance and retention of the important memories and skills.

When we sleep, our mind plays back the important memories and skills we wish to retain. This appears to be one of the reasons why we dream, and why our dreams are influenced by our previous actions. This consolidation also prunes the junctions and neuronal links for memories and tasks we don’t use, ones that are not useful to us. This constant pruning hones our brain to keep only what we require. 



Most people need about 8 hours of sleep a night. This differs between people and also by age. Not achieving the required amount can make a person tired and irritable throughout the day. But if this is maintained for long periods, it can have serious negative effects on your ability to concentrate. This in turn can lead to poor performance at work, increasing stress, and make you prone to accidents.

Whilst we have spoken about why your brain needs sleep; your body needs it too. Without it, you can become more prone to medical conditions such as obesity, heart disease, high blood pressure and diabetes. It even has noticeable effects on your fertility, by reducing the secretion of reproductive hormones.

Sleep is an interesting topic and one we hope to cover again in the future. There is a lot to learn about sleep, and a lot the medical community still needs to discover regarding its uses. From this blog, you should have at least learnt about the basics of sleep, its critical importance as well as why your brain needs it so much.