A summary of sleep

Hello everyone and welcome to this week's blog post. This article is the first in a two-part series about sleep. In this article, I will explain what sleep is, how it is regulated and the stages of sleep. I will then highlight some of the diseases associated with sleep. Next week, I will analyse sleeping pills and determine how effective they are.

Sleep is incredibly important for the human body. It is essential for forming and maintaining pathways in the brain, creating new memories and allows us to feel refreshed the next morning. Additionally, new evidence has suggested that sleep also allows toxins (which build up during the day) to be flushed from the brain.

Sleep is controlled by many structures in the brain. The hypothalamus, which is located at the base of the brain, is the region of the brain responsible for switching the body between being asleep and being awake. This is because it has clusters of cells (called the suprachiasmatic nucleus) which detect light levels using information received from the eyes. When it is dark, the hypothalamus then sends signals to other parts of the brain to promote sleep.

One of these regions is the pineal gland. Once it receives signals from the suprachiasmatic nucleus in the hypothalamus, it is stimulated to secrete the hormone melatonin, which helps you go to sleep.

Chemical structure of melatonin

Over time, the body becomes used to this pattern of light and dark. This is why, if a person stays inside an underground bunker for 24 hours, melatonin production will still increase at a similar time to usual and they will still feel sleepy at the time they usually go to sleep, even though the eyes can't detect natural light. This phenomenon, which is called the circadian rhythm, also helps explain jet lag, as your internal circadian clock is out of sync with the light levels your eyes are detecting.

The hypothalamus communicates with the brain stem to coordinate the transition between being asleep and being awake. This is mainly achieved using a hormone called GABA (gamma-Aminobutyric acid), which is secreted by the brain stem. This hormone acts on brain centres associated with arousal and reduces their activity, thus making you feel sleepy. In addition, the brain stem is also responsible for paralysing your muscles during REM sleep, so that you don't hurt yourself during vivid dreams.

Chemical structure gamma-Aminobutyric acid

Furthermore, the thalamus (a small structure located above the brain stem) is also involved in sleep. The thalamus's primary function is relaying information between the sensory organs and the cerebral cortex (which interprets the information received and moves information from short-term to long-term memory. During REM sleep, the thalamus plays a crucial role in providing images and sounds during vivid dreams.

There are four stages of sleep, which repeat in cycles during the night. Generally, each cycle lasts around 90-110 minutes, while each stage lasts between 5 -15 minutes. The first 3 stages of sleep are all part of non-rapid eye movement sleep (NREM), while the fourth stage is rapid eye movement sleep (REM).

Simplified diagram of the phases of sleep

Stage 1, which is part of NREM sleep, is a transitional phase between being awake and asleep. During this stage, most people sleep very lightly so can be easily woken up by sounds or movements in the environment around them. Additionally, muscle tension decreases and eyes begin to move in slow rolls.

In stage 1, the brain also produces low-frequency waves called alpha waves and theta waves. These brain waves, when viewed on an EEG, are very similar to someone who is resting, yet awake. Indeed, many people who are woken during stage 1 sleep often do not realise that they fell asleep at all.

In stage 2, the body relaxes further and awakenings do not occur as easily. This is partly because of sleep spindles, which are high-frequency brain waves that occur in rapid bursts. In addition, K-complexes are also present. These are high amplitude brain waves which are specifically associated with stage 2 sleep. Scientists believe sleep spindles and K-complexes help to protect the brain from waking up during sleep. Moreover, core body temperature and heart rate decrease during this stage, while the slow-moving eye rolls also stop.

During stage 2 sleep, memory consolidation occurs. This is where the brain codes the information it acquired during the day, essentially moving memories from short-term memory to long-term memory, thus allowing you to recall information at a later date. In addition, synaptic pruning also occurs in this stage, where the brain removes synapses (connections between neurones) that have been weakened from disuse. Synaptic pruning is essential to allowing us to learn new information and allows the brain to remain efficient, as old synapses can be replaced by higher quality connections.

Stage 3 is the deepest stage of sleep. In this stage, the brain waves have very low frequencies and it is much harder to be woken up. Moreover, sleepwalking can sometimes occur in this stage.

This stage of sleep is the stage required to feel refreshed in the morning - a study in 2008 showed that those who don't receive enough stage 3 sleep did not feel refreshed in the morning, even if they had slept for the recommended 7-9 hours. This study clearly underlines how the quality of your sleep can be just as important as the length of time you sleep in affecting how you feel the next morning.

During REM sleep, which is the fourth stage of sleep, the brain waves are similar to someone who is awake. The eyes also move rapidly from side to side, thus giving this stage its name. In addition, the most vivid dreams occur during this stage. Breathing also becomes faster and heart rate and blood pressure both increase.

In this stage, the neurotransmitters gamma-aminobutyric acid (GABA) and glycine are both released. These neurotransmitters paralyse all muscles in the body, with the exception of those involved with breathing, circulation and the eyes. It is thought that is is a self-defence mechanism which stops you from hurting yourself during vivid dreams.

If you don't get enough sleep, then research suggests that you may be at a higher risk of developing conditions such as diabetes, high blood pressure, obesity, depression and cardiovascular diseases. Moreover, after not having enough sleep for a prolonged period of time, people generally find it much harder to concentrate on tasks, which increases the risk of accidents at home and on the road.

There are many diseases associated with sleep. For example, sleep apnoea is a condition which occurs when the airway becomes too narrow during sleep. If this occurs in someone then they may temporarily stop breathing and start choking, before waking up. In addition, sleep apnoea can also lead to other conditions such as high blood pressure, stroke and depression. Further, sleep apnoea could also decrease the quality of your sleep, making it much harder to focus. Common risk factors of sleep apnoea include having a thicker neck, smoking, drinking alcohol and having large tonsils.

Sleep paralysis is a scary, yet harmless, condition associated with sleep that affects as many as 4 in 10 people. Sleep paralysis has several causes including insomnia, disrupted speech patterns, PTSD, and panic disorders. The main symptom of this disease is where someone will wake up, as their brain is active, but be unable to move as their muscles are paralysed. Fortunately, according to the NHS, most people who do suffer from this disease only experience these episodes a couple of times during their lifetimes.

On the flip side, getting enough sleep has many benefits for your health, including boosting your immunity, increasing mental wellbeing and reducing the risk of increasing diabetes. In addition, according to the NHS, it also increases the levels of the ghrelin hormone, which helps people feel full after eating and thus helps people lose weight. Moreover, getting sufficient sleep also helps decrease blood pressure and inflammation, thus decreasing the risk of heart disease.

Sources:

https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep

https://www.sciencedirect.com/topics/neuroscience/sleep-waking-cycle

https://www.medicinenet.com/script/main/art.asp?articlekey=8684

https://www.nhs.uk/live-well/sleep-and-tiredness/why-lack-of-sleep-is-bad-for-your-health/

https://courses.lumenlearning.com/wsu-sandbox/chapter/stages-of-sleep/

https://www.dreams.co.uk/sleep-matters-club/synaptic-pruning-sleep/

https://www.sciencedaily.com/releases/2012/07/120711131030.htm

https://www.tuck.com/stages/

https://www.wakehealth.edu/Specialty/s/Sleep-Medicine/Sleep-Explained

https://www.nhs.uk/conditions/sleep-apnoea/

https://www.nhs.uk/conditions/sleep-paralysis/

https://www.webmd.com/sleep-disorders/sleep-paralysis