Sleep is an important part of our day and on average a person spends 26 years sleeping! This seems like a large waste of time. However, we should be getting 8 hours sleep every day and adolescents should be getting 10 hours sleep. Those that get less than 7 hours are considered sleep deprived. As a teenager, I stay up late working or watching television. To me just saying that you need X hours of sleep doesn’t make me suddenly change my ways and start to go to bed earlier. Instead, I am going to try to convince you to sleep more by explaining these things:
- The stages of sleep
- The science of sleep
- The effects sleep has on our physical health
- The effects sleep had on our mental health and brain function
This is quite a long post so feel free to jump to the parts you want to read. I have tried to divide the sections up so it is easier to navigate.
The stages of sleep
Sleep is split up into 2 main stages – NREM and REM. NREM meaning non- REM. REM meaning Rapid Eye Movement.
Within these are four distinct stages:
- Stage 1 (NREM)
- Stage 2 (NREM)
- Slow – wave sleep (NREM)
- REM sleep
Stage 1 is around 1-5 minutes and this is when you are dozing off. It is very easy to wake up in this stage as your body hasn’t shut down just yet and your brain is still functioning.
Stage 2 lasts around 10-25 minutes. During this time your body temperature will drop, brain activity will decline, and your heart and respiration rates will slow down.
Stage 3 is known as slow-wave sleep. This is because this stage is identified by its pattern of delta waves. In this stage your muscles will relax even more and your memory will begin to be formed. The declaration memories (i.e recalling facts for exams) from that day are stored temporarily. Hence it is important to get a good nights sleep before an exam instead of staying up late ‘cramming’.
During REM sleep our bodies essentially shut down. Sometimes this is referred to as temporary paralysis, with the exception of our eyes and breathing muscles. REM is the deepest of the stages and it is when we have our intense dreams (the other stages include dreams also). However, unlike the other stages brain activity picks up which is why we have these dreams that are often so vivid. This is also when our procedural memory (i.e the memories of how to play an instrument) are formed. Scientist recommend you practice an instrument 1 hour before you go to sleep which will maximise your memory of what you have learnt.
Sleep is separated into 90 minute intervals where the stages of sleep alternate. The time spent in each stage of sleep varies depending on what interval you are in.
The science of sleep
Now you know the stages of sleep lets find out the neuroscience behind this all…the interesting part!
There is no doubt you have heard this term before, specifically in terms of sleep. A circadian rhythm is a 24hour clock that regulates a biological process, there are lots of different ones. But the one we are concerned about is the sleep-wake cycle.
The rhythm is your body’s internal clock to tell you when to sleep and when to wake up. This is all synchronised by the master clock which is often referred to as the circadian pace maker in the hypothalamus of the brain. The master clock will send signals to indicate it is time to sleep depending on the environment. If there is light, alertness will be generated. If it is dark (signalling night) the master clock produces melatonin (see below for more details). The blue light from our devices disrupts melatonin and confuses the system into thinking it is light, therefore, we won’t sleep as easily.
The two main chemicals in this sleep-wake cycle are: adenosine and melatonin. Both help to slow your heart rate and make your muscles sleep to encourage you to sleep.
Melatonin is a natural hormone produced by the brain. It effects receptors in the body to encourage you to sleep. It is important for synchronising the circadian rhythm because it is effected by the light levels.
Adenosine is a neurotransmitter chemical. Cells break down Adenosine Triphosphate (ATP) for energy which breaks down into the waste product: adenosine. Increases in physiological activity (which requires energy)means there is a higher concentration of adenosine in the bloodstream, which will eventually reach the brain. The adenosine will bind to adenosine receptors to cause a sleep pressure where you feel drowsy and tired. There are also two types of receptors. If adenosine binds to an A1 receptor they make neurones that promote alertness less active. If adenosine binds to the A2A receptor they make neurones that promote sleep more active. Combined, the adenosine encourages sleep.
Photo Credit: Science Learning Hub
Caffeine actually suppresses adenosine receptors so that it can’t signal to the body that its time to sleep. The structure of caffeine and adenosine are similar so caffeine can also bind to the adenosine receptors. However, the structure is not exactly the same. So although caffeine binds to the receptors, it does not activate them into stimulating sleep. They block the receptors so adenosine can’t bind to them. In this way, caffeine doesn’t reduce the your need to sleep just blocks the signal that tells you to sleep.
Photo Credit: Sleepopolis
Caffeine is not good for you in the long term. This is because as you drink coffee your body will respond by creating more adenosine receptors. There will be free receptors that the caffeine can’t block as there aren’t enough of them. This means that the caffeine dose needs to increase in order to have the same effects of blocking adenosine receptors. You will then need to drink more caffeine in order to keep you awake, leading to an addiction and reliance on caffeine to stay awake.
Factors that effect your sleep
There are a few factors that effect sleep patterns and your circadian rhythm:
- How much your body needs to sleep – regarding adenosine levels and how long you have been awake
- Stress – stress hormones like adrenaline will keep you awake and can even cause acute insomnia. The adrenaline causes your heart rate to increase rather than decrease and will cause you to ‘crash’ as your body gets very tired. Hence, your circadian rhythm is disrupted.
- Hunger – stomach pains will keep you awake
- Light levels – melatonin responds to the environment. Blue light from electronic devices make it more difficult to sleep because your internal body clock believes it is day time.
The effects of sleep on our physical health
Now we know what makes us sleep but what does sleep actually do that makes it so important? There are many health benefits of sleeping and many links between sleep and medical problems.
Cardiovascular benefits are an important part of sleep that helps reduce the risks of developing heart disease, high blood pressure or having a stroke. This is because sleep is responsible for healing and repairing your heart and blood vessels. During the NREM the heart rate slows and blood pressure drops. As a result, there is less stress on the heart that allows it to rest from the activity during the day. Atherosclerosis, a build up of plaque in the blood vessels, can be prevented by a good amount of sleep. This is because the sleep helps to regulate hypocretin production in the hypothalamus of the brain which means there is less build up of calcium.
Sleep can also reduce the chance of becoming obese because it suppresses hunger. Without enough sleep the body will make more ghrelin and less leptin which makes you more hungry. Indirectly, sleep helps to reduce the risk of other cardiovascular problems too by limiting obesity.
The immune system is also significantly effected by sleep. This is an important part of our bodies which we need to fight infections and viruses like COVID-19. During sleep the energy use for muscles and breathing decreases so more energy is available for the immune system to use. With this energy the immune system will produce cytokines (chemicals that initiate inflammation). Inflammation is important for repairing cells and tissues and defending against viruses and bacteria. Studies have actually shown that the lack of sleep can make people more susceptible to colds and flus.
In the long term, sleep can be beneficial to your health. Rare days of late nights is fine but a consistent lack of sleep will damage your body a large amount.
The effects of sleep on mental health and brain function
17 year old Randy Gardner, in 1963, stayed awake for 11 days (a world record at the time) for a science project. His friend recorded his health during this time. He said that there was little short term effects on Gardner’s physical health. There were, however, behavioural changes. He became moody and irritable, found it difficult to concentrate and experienced hallucinations. There were significant problems with his short term memory. Hence, his cognition was disrupted. Obviously, this is an extreme case with no sleep at all but over a lifetime, a lack of sleep can build up. Mental health problems can range from depression and anxiety to chronic stress. Limited brain function can lead to Alzheimers/dementia in the long term to just poor memory for an exam the next day in the short term.
People with depression often find that sleeping problems accompany their other problems but sleep isn’t just a consequence of depression. Lack of sleep can exaggerate depression which creates a vicious cycle between the two (sometimes this is referred to as a bidirectional relationship). Sleep causes changes in serotonin levels. Serotonin levels need to remain relatively stable to ensure there aren’t mood swings: whether from calm to angry or from happy to sad. Serotonin makes someone happy whilst a lack of it makes them sad. Depression can be caused by rapid changes in these levels: where they can be very happy and suddenly become extremely sad.
Anxiety can have similar effects to stress in disrupting sleep but lack of sleep can worsen it. Gardner became more paranoid and stressed with a lack of sleep. Anxiety can also make your REM sleep dreams more troubling and reinforce a fear or worry that was already there. Like depression, the two have a bidirectional relationship.
Sleep deprivation is known to increase levels of an Alzheimer’s protein called beta-amyloid. Beta-amyloid clumps together and creates plaques between neurones. As a consequence, the transmission between neurones is disrupted which reduces memory and brain function. Sleep works to clear the accumulation of waste products with its own glymphatic system which drains out beta-amyloid. Without sufficient sleep, there are higher levels of beta-amyloid and the plaques that they form. Furthermore, dementia has also been linked to poor blood-flow to the brain. Sleep helps to clear blood vessels from plaque. Hence, without sleep there is reduced flow of blood.
Learning is facilitated by sleep because our memory improves. As mentioned before, concentration is effected by sleep, hence the ability to focus on work is reduced.
Memory is also effected. Slow-wave sleep is involved in declarative memory that is important for processing important and complex information. Procedural memory is forms in REM sleep. Sleep helps to consolidate the information learnt during the day. This makes recall a lot easier when taking an exam or just as general knowledge. How does memory actually work? Well that is a complicated question. In short, there is a storage space for memories where there are synapses (gaps between neurones). A weak or strong synapse will determine how well you remember something. Deep REM sleep is where you solidify connections to information. In REM sleep your brain will replay information. More replays mean stronger synapses which means a stronger memory.
Photo Credit: Khan Academy
Here you can see two neurones. The gap between them is called the synapse. For nerve impulses to be sent around the body they first have to cross the synapses.
Photo Credit: ResearchGate
This is a synapse up close. The neurotransmitters are chemicals that diffuse across the synapse to trigger an electrical impulse. The electrical impulse can’t jump from one neurones to the other so neurotransmitters mediate this (this is quite a complicated process so if you want to know more you can do some extra reading). This also links to the adenosine mentioned earlier – you can see the adenosine (neurotransmitter) and the adenosine receptors (receptors) on the diagram.
Sleep is important and you should get AT LEAST 7 hours. Try to get some sleep instead of working later into the night – in the long term you will be grateful for the choice you have made. Maybe you might even appreciate sleep so much that you dream of sleeping.