The Science of Sleep – What we know about Sleep

Welcome back to second part our exploration into sleep and its crucial role in both everyday function and overall health. In this section, we’ll dive into the science of sleep, gaining insights into how the sleep process functions and unraveling the mysteries that lead to improved sleep quality. 

Table of Contents


In our previous post, we highlighted the fundamental importance of sleep for overall well-being. Sleep serves as a time for restorative processes, strengthening the immune system, promoting muscle recovery, and enhancing cognitive function. It aids in memory consolidation, improves attention and problem-solving skills, and balances our emotional well-being. Prioritising healthy sleep patterns can boost emotional resilience, mood, and stress resilience while preventing irritability and mood swings. For us gym goers, if we don’t have optimal sleep this can mean missing training or seriously impaired performance during workouts. Not good.

In this post we shall look into the science of sleep.  The things that go on behind the scenes in your brain that prompt you when to sleep and when to wake.  We’ll uncover the precise mechanisms that underlie the remarkable benefits of a good night’s rest.

FMRI studies. Sleep and the brain. Neuroscience. The science of sleep.

Contrary to the common perception of rest, the brain is remarkably active during our sleep, engaging in intricate processes that contribute to our overall well-being. In sleep studies, scientists use functional magnetic resonance imaging (fMRI) to unveil the brain’s activity during sleep. 

The aim is to give you a straightforward understanding of how it all works and find out what can stop those processes running effectively.  By unravelling the mysteries behind sleep, we aim to equip you with knowledge that sheds light on potential obstacles and pitfalls.  Granted, we are not neuroscientists, so we’ll aim to break down the jargon heavy and keep things as straightforward and to the point as possible.


Sleep is a complex physiological process that is influenced by various biological mechanisms, these include:

  1. Circadian Rhythms:
  2. The Role of Neurotransmitters:
  3. Sleep-Wake Homeostasis:
  4. Understanding Sleep Patterns and Cycles:

Circadian Rhythms

The Body Clock

Circadian rhythms are the body’s internal clocks that regulate our sleep-wake cycle.  They influence various physiological processes (including sleep, metabolism, and hormone release ) over a 24-hour period.  These rhythms are primarily driven by our exposure to natural light and darkness, which signals to our brain when it’s time to be awake and when it’s time to wind down.

They influence not only when we feel awake or sleepy but also impact body temperature, hormone production, and even cognitive performance. Disruptions to our circadian rhythms, such as jet lag or irregular sleep patterns, can lead to sleep problems and affect overall well-being. Understanding and respecting these rhythms can lead to better sleep quality and enhanced daily functioning.

Optimum Functioning

In an ideal world, the circadian rhythm will naturally rise in the early morning, promoting wakefulness and alertness, and will reach a peak in the evening. After a waking period of around 15 hours the pressure to sleep becomes greater and greater, in other words, we get tired. With the onset of darkness, the circadian rhythm drops to the lowest level and helps to maintain sleep. In the modern world there are factors that can improve and disrupt these circadian rhythms, these include our exposure to natural light and blue light (from screens/phones etc).

When a circadian rhythm is working well, you will notice you function at peak ability, when its out of sync…well things don’t go so well…

More on Circadian Rhythms - (Click on heading to open)

  • The term “circadian” comes from the Latin words “circa” (around) and “diem” (day), reflecting the approximately 24-hour cycle of these rhythms.
  • Circadian rhythms are driven by a group of specialized cells in the hypothalamus of the brain called the suprachiasmatic nucleus (SCN).
  • Light is the primary external cue that synchronizes circadian rhythms to the 24-hour day-night cycle.
  • Photoreceptor cells in the retina of the eye (specialized cells that are sensitive to light, particularly in the blue spectrum) send signals to the SCN, signaling whether it is daytime or nighttime. Exposure to bright light in the morning helps reset the internal clock.
  • One of the most well-known circadian rhythms in humans is the daily variation in core body temperature. It tends to be lowest in the early morning hours and highest in the late afternoon and early evening.
  • The temperature fluctuations are closely linked to the sleep-wake cycle and the body’s energy expenditure throughout the day. During the early morning hours, the body temperature tends to be lowest, facilitating restful sleep. As the day progresses, the temperature gradually rises, reaching its peak in the late afternoon and early evening, promoting alertness and performance.
  • The sleep-wake cycle (see below) is tightly linked to circadian rhythms. The SCN plays a significant role in regulating when we feel alert and when we feel sleepy.
  • Circadian rhythms influence the release of the hormone melatonin, which helps regulate sleep. Melatonin levels typically rise in the evening, promoting sleepiness, and drop in the morning, promoting wakefulness.
  • Hormones are chemical messengers that play a crucial role in regulating various bodily functions.  They influence processes such as metabolism, growth, stress response, and overall well-being. Their precise timing and balance are essential for maintaining optimal health and functioning.
  • Many hormones, such as cortisol, growth hormone, and insulin, follow circadian patterns. These rhythms can affect metabolism, energy levels, and overall health.
  • Misalignment of hormonal rhythms due to disrupted circadian rhythms can contribute to health problems like metabolic disorders such as insulin resistance and obesity.
  • Circadian rhythms impact mood and cognitive performance through their influence on hormone secretion. For example, heightened alertness often corresponds with daytime when cortisol levels peak, while lower alertness tends to coincide with nighttime when cortisol is reduced.
  • Disruptions to circadian rhythms, such as shift work or jet lag, can lead to mood disturbances and impaired cognitive function and mood fluctuations.
  • While the average human circadian rhythm is around 24 hours, there is considerable variation among individuals. Some people are “morning people” (morning chronotypes), while others are “night owls” (evening chronotypes).
  • These individual differences are partly genetic and can impact sleep patterns and daily functioning.
  • Maintaining a stable circadian rhythm is crucial for overall health. Disruptions to these rhythms have been associated with various health issues, including sleep disorders, mood disorders, and metabolic disorders.
  • Properly aligning your circadian rhythms with your daily activities, known as “circadian alignment,” can lead to improved well-being.


What is a neurotransmitter?

A neurotransmitter is a chemical messenger in the brain and nervous system that transmits signals between nerve cells, influencing various functions. In the context of sleep, these molecules are pivotal in regulating the sleep-wake cycle and determining the quality of our slumber. For example, Serotonin promotes wakefulness during the day, while melatonin guides our transition into restful sleep at night. Neurotransmitters play a significant role in shaping our sleep patterns and overall sleep quality.  (Think of the brain sending a text message to the body in the evening, to say it’s time for bed).

Neurotransmitters also play a significant role in regulating the various stages of sleep and wakefulness. Several neurotransmitters influence the transitions between wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep (see below).

Key neurotransmitters involved in the sleep process - (Click on heading to open)

Serotonin is a neurotransmitter associated with wakefulness and alertness. It helps keep us awake and focused during the day. Low serotonin levels are linked to sleep disorders such as insomnia and disrupted sleep patterns.

Norepinephrine, also known as noradrenaline, promotes wakefulness and arousal. It is released by the brainstem and contributes to our ability to stay awake and attentive.

Adenosine is another key player in sleep processes. Adenosine is a neuromodulator that accumulates in the brain during wakefulness and promotes sleepiness. Its levels increase the longer we stay awake, and it contributes to the homeostatic regulation of the sleep-wake cycle. Adenosine receptors in the brain are activated by adenosine, inducing drowsiness and promoting the initiation of sleep. Therefore, adenosine is an integral component in the neurochemical regulation of sleep.

Acetylcholine plays a role in REM sleep. During REM sleep, the brain becomes highly active, and acetylcholine is released, leading to vivid dreams and increased brain activity. A deficiency of acetylcholine can lead to REM sleep disturbances.

GABA is an inhibitory neurotransmitter that has a calming effect on the brain. It reduces neural activity, helping to initiate and maintain sleep. Medications that enhance GABA activity are often used to treat insomnia and sleep disorders.

Glutamate is an excitatory neurotransmitter that promotes wakefulness. During NREM sleep, there is a decrease in glutamate activity, contributing to the transition from wakefulness to deeper stages of sleep.

Dopamine plays a role in regulating the sleep-wake cycle, particularly in the regulation of the circadian rhythms that influence when we feel alert and when we feel sleepy.

Histamine is primarily associated with wakefulness. Medications that block histamine receptors are often used to promote sleep and treat sleep disorders.

Melatonin is a hormone, not a neurotransmitter, but it plays a critical role in regulating the sleep-wake cycle. The pineal gland in the brain produces melatonin in response to darkness, signaling the body that it’s time to prepare for sleep.

Orexin is a neurotransmitter that promotes wakefulness and alertness. A deficiency of orexin is associated with narcolepsy, a sleep disorder characterized by sudden and uncontrollable daytime sleepiness.

The intricate balance of these neurotransmitters helps orchestrate the transitions between wakefulness, different stages of sleep, and REM sleep.

Circadian Rhythms and Neurotransmitters

Circadian rhythms and neurotransmitters work hand in hand to shape our sleep-wake cycles. These internal biological rhythms, controlled by the brain's suprachiasmatic nucleus, impact the release and activity of important neurotransmitters. For example, melatonin, a sleep-promoting neurotransmitter, is produced in response to the circadian rhythm's night time signals. Simultaneously, neurotransmitters like serotonin and adenosine, which regulate mood and alertness, are influenced by these rhythms, shaping our periods of wakefulness. This intricate interplay between circadian rhythms and neurotransmitters ensures a synchronised rhythm for our daily sleep, preparing the body for restful nights and wakeful days.

Key neurotransmitters involved in the sleep process:

Problems with neurotransmission can disrupt sleep. (Back to the texting analogy, this is like the brain texting the body to sleep, but there is no phone signal reception. So the message does not get through.)  Factors like stress and anxiety overactivate norepinephrine and cortisol, hindering sleep’s calming effects. Alcohol and substances, while inducing drowsiness, interfere with neurotransmitter balance. 

Neurotransmission (the communication process within our nervous system) plays a pivotal role in relaying messages crucial for our sleep. As electrical impulses travel through nerve cells (left), they are converted into neurotransmitters and passed onto other nerve cells to relay messages. A well-functioning nervous system is essential for orchestrating the sleep processes seamlessly.  Unfortunately there are many external factors that can negatively affect/hinder these processes. Meaning, we find ourselves unable to ‘switch off’.  

Some medications, especially antidepressants and stimulants, can disrupt the sleep-wake cycle. Neurological disorders and sleep disorders like insomnia or narcolepsy impact neurotransmitter function. Dietary choices affect serotonin synthesis. Age-related changes and disruptions to the sleep-wake cycle also impact neurotransmitter regulation, showcasing the complex interaction between neurotransmitters and lifestyle, environmental, and health factors affecting sleep.

Sleep-Wake Homeostasis

The third factor of our look into the science of sleep is ‘Sleep-wake Homeostasis*’.  Sleep-wake homeostasis is a physiological mechanism that regulates our sleep patterns and ensures that our need for sleep is balanced. This self-regulating process acts like a “sleep debt” or “sleep pressure” mechanism, which accumulates the longer we are awake and dissipates during sleep. As we stay awake, our bodies accumulate adenosine, a neurotransmitter that builds up in the brain and contributes to a growing need for sleep. The longer we’re awake, the more the adenosine levels builds up, creating a mounting pressure for rest.

Tired. Adenosine levels. The Science of Sleep.

Uh oh! Someone’s adenosine levels are too high! This guys been skimping on sleep.

During sleep, (particularly deep or slow-wave sleep), the body works to clear out this sleep debt by processing and eliminating adenosine. As a result, we wake up feeling refreshed and alert, with the sleep debt essentially paid off. However, if we consistently deprive ourselves of sleep or have interrupted sleep, this adenosine “debt” accumulates and can lead to feelings of tiredness, grogginess, and impaired cognitive and physical function during the day. Sleep-wake homeostasis is a key element in regulating the balance between wakefulness and the need for rest, helping us maintain a healthy sleep pattern.

*Homeostasis is any self-regulating process living organisms use to maintain stability while adjusting to conditions that are best for its survival.  For example, feeling thirsty prompts us to drink water, restoring the balance of hydration. If we drink excessively, our bodies prompt us to use the toilet, again the balance is restored.

Simple Breakdown of Sleep-Wake Homeostasis:

  1. Adenosine Accumulation: When you’re awake and active, your body uses energy, and adenosine is a natural byproduct of this energy consumption. Adenosine builds up in your brain over time, like a fuel gauge that fills as you go about your day.

  2. Sleep Pressure: Adenosine acts as a signal, creating what scientists call “sleep pressure.” Think of it as a gentle push, urging you to go find a comfy pillow. As adenosine levels rise, so does this sleep pressure, making you increasingly drowsy.

  3. Sleep Reset: Sleep-Wake Homeostasis works to reset this adenosine accumulation during sleep. When you sleep, your brain gets busy clearing out the excess adenosine, reducing the sleep pressure.

  4. The Sleep Debt: Problems occur if you regularly shortchange yourself on sleep, perhaps due to work, stress, or late-night Netflix binges, the adenosine keeps piling up. You accumulate what’s known as a “sleep debt.” It’s like spending more than you earn – eventually, you’ll have to repay it.

  5. Repayment Demands: Your body isn’t too happy about unpaid sleep debts. It’ll demand repayment by making you feel extra tired during the day. This is your body’s way of saying, “Hey, we need more sleep to clear this adenosine debt.”

The above charts demonstrate Sleep-Wake Homeostasis. The bodily mechanism that ensures you get the right amount of sleep needed to maintain your body’s balance.   The blue line on both charts represents the adenosine build up.  The top chart shows the build up during a day without napping, whilst the bottom chart shows the impact of napping during a day. Both clear out adenosine and wipe the sleep debt.

Sleep Patterns and Cycles:

Phew! So we have finally managed to actually get to sleep.  Now we shall shift our look into the science of sleep into the structure of sleep itself.  Sleep is organised into distinct sleep stages or ‘cycles’. These cycles each serve a unique purpose in the process of sleep. By grasping the sequential patterns of these sleep stages, we gain a deeper understanding of the science of sleep and the mechanisms at play during these stages..

Understanding Sleep Patterns and Cycles:

Sleep cycles consist of distinct stages, each serving its unique purpose in promoting physical and mental well-being.  It can be loosely divided into REM and NREM sleep,

No, not this REM…

Non-rapid Eye Movement (NREM) Sleep

Non-rapid eye movement (NREM) sleep is divided into three stages: N1, N2, and N3. 

  • N1 = This is the lightest stage of sleep and acts as a transitional phase between wakefulness and deeper sleep.
  • N2 = This is a deeper sleep stage where most of our sleep occurs. It’s characterised by a decrease in heart rate and body temperature.
  • N3 = also known as deep sleep or slow-wave sleep, is the stage when the body undergoes essential restorative processes, such as tissue repair, hormone regulation, and immune system strengthening.

Rapid Eye Movement (REM) Sleep:

After progressing through NREM stages, we enter REM sleep, which is characterized by rapid eye movements, heightened brain activity, and vivid dreaming.

  • REM sleep is a stage associated with vivid dreaming and heightened brain activity.
  • During REM sleep, our brains simulate experiences and consolidate memories, supporting cognitive function and emotional regulation. 
  • During this stage, our muscles become temporarily paralyzed, preventing us from acting out our dreams.

As the night progresses, sleep cycles follow a predictable pattern, with longer periods of deep NREM sleep occurring earlier in the night and more frequent REM sleep episodes toward morning. This cyclical nature of sleep stages ensures a balanced and restorative sleep experience, enabling us to wake up feeling refreshed and energised.

Sleep Cycles

The progression of a sleep cycle usually goes like this:

  1. NREM Stage 1 (N1).
  2. NREM Stage 2 (N2).
  3. NREM Stage 3 (N3).
  4. REM Sleep.

Throughout the night, we cycle through these stages multiple times, with each sleep cycle lasting around 90 to 120 minutes on average. The proportion of time spent in each stage can vary throughout the night, with the amount of REM sleep increasing and N3 sleep decreasing in later cycles. This cycling through different sleep stages is essential for various aspects of physical and mental well-being.

When things go wrong

In an ideal world, these biological mechanisms work together to create a rhythmic sleep-wake cycle. When these mechanisms function properly, they ensure that you get the right amount and quality of sleep your body needs for physical and mental health. Disruptions in these mechanisms, such as irregular sleep patterns or imbalances in neurotransmitters, can lead to sleep disorders and affect overall well-being.

Sleep cycle problems are quite common, affecting a significant portion of the population. Issues like insomnia, delayed sleep phase syndrome (DSPS), and irregular sleep-wake rhythm disorder can disrupt natural sleep patterns. Factors such as age, lifestyle, and work schedules contribute to the prevalence of these problems. While occasional disruptions are normal, persistent issues may require attention. Lifestyle adjustments and improved sleep habits can often help address these challenges, promoting better overall sleep health.

Factors that Disrupt the Body's sleep Mechanisms

Now we have covered the science of sleep, let us examine the external factors that negatively impact/influence these processes.  In 21st century life, everyday influences and behaviours we rely on heavily in our hectic schedules can inadvertently disrupt our sleep quality.  Stress, prolonged screen exposure, and overuse of stimulants stand out as common obstacles, intricately interfering with the delicate mechanisms orchestrating our nightly rest. By understanding the disruptive influence of some of these obstacles we can begin to look at devising strategies to alleviate their impact and elevate the overall quality of our sleep.

Common factors that disrupt our sleep

When you’re stressed or anxious, your body releases stress hormones like cortisol, which can increase alertness and hinder the ability to fall asleep. This emotional turmoil can also lead to racing thoughts and worry, making it challenging to relax and enter a restful sleep.

Exposure to screens emitting blue light, such as smartphones, tablets, and computers, before bedtime can suppress the production of melatonin, a hormone that regulates sleep. This disruption to your circadian rhythm can lead to difficulties falling asleep.

Consuming caffeine or other stimulants, especially later in the day, can interfere with your ability to fall asleep. These substances stimulate the central nervous system and keep you alert, making it difficult to relax for sleep. While alcohol may make you feel drowsy initially, it can negatively affect the quality of your sleep. It can disrupt the sleep cycles, leading to more fragmented and less restful sleep. Additionally, some medications, like certain antidepressants or decongestants, can interfere with your sleep. They may have stimulating effects or disrupt the balance of neurotransmitters involved in sleep regulation. It’s important to be mindful of the substances and stimulants you consume, considering their potential impact on your sleep patterns.

Noise disturbances during sleep can activate the body’s stress response and increase cortisol levels. These physiological reactions can disrupt sleep patterns and lead to reduced sleep quality. Additionally, factors such as uncomfortable bedding, excessive noise, or extreme temperatures can impact your sleep environment. These discomforts can disrupt your sleep and result in awakenings during the night. Creating an optimal sleep schedule and environment involves addressing both internal and external factors to promote restful and uninterrupted sleep.

Inconsistent sleep patterns can disrupt your circadian rhythms and sleep-wake homeostasis, causing your body to become confused about when it should be awake or asleep. This inconsistency can lead to sleep difficulties and reduced sleep quality.

Conditions that cause chronic pain, such as arthritis or fibromyalgia, can disrupt sleep by causing discomfort and frequent awakenings during the night. Moreover, physical issues like pain from injuries, chronic conditions, or uncomfortable sleeping positions can disturb your sleep. Discomfort can lead to frequent awakenings and difficulty falling back asleep.

Consuming heavy meals or spicy foods close to bedtime can cause discomfort and indigestion, making it harder to fall asleep. Additionally, lying down immediately after eating may contribute to acid reflux.

Conditions like insomnia, sleep apnea, restless leg syndrome, and narcolepsy can severely impact sleep. These disorders disrupt the sleep cycle, resulting in insufficient or fragmented rest. Additionally, mental health plays a crucial role in sleep patterns. Conditions like depression and anxiety can result in altered neurotransmitter balance, impacting sleep patterns. These mental health issues can lead to difficulties falling asleep or staying asleep. The intricate relationship between sleep disorders and mental health underscores the importance of addressing both aspects for comprehensive well-being. Effective management often involves a holistic approach that considers both the physical and psychological factors influencing sleep.

Shift work disrupts the body’s natural circadian rhythms, making it challenging to maintain a consistent sleep schedule. This can lead to sleep disturbances and an increased risk of sleep disorders.

While short naps can be beneficial, long or irregular daytime napping can affect your ability to fall asleep at night. This can disrupt your sleep-wake cycle and lead to insomnia. Additionally, intense physical activity close to bedtime can raise body temperature and stimulate the release of adrenaline. This can make it difficult to relax and fall asleep quickly.

Discontinuing certain medications, particularly those that affect neurotransmitters, can result in withdrawal symptoms, including sleep disturbances.


Having a good sleep relies on the smooth teamwork and functioning of your internal clock (circadian rhythms), brain messengers (neurotransmitters), a built-in rest metre (sleep-wake homeostasis), and a sleep cycle with different stages. Together, they make sure your body knows when it’s time to be awake and when it’s time to rest deeply. Problems can pop up if your lifestyle or surroundings throw this teamwork off balance.

To keep your sleep in harmony, it’s useful to grasp how these processes work together. Paying attention to your daily habits and the environment you’re in can help create a sleep-friendly space and prevent issues that might disturb the delicate balance of your sleep system.

In the final instalment of our exploration into the world of sleep, we will dive into practical tips and strategies aimed at enhancing sleep quality and overcoming common obstacles that disrupt our nightly rest.


  1. National Sleep Foundation: The National Sleep Foundation’s website provides a wealth of information on sleep, including articles on sleep health, sleep disorders, and sleep research. 

  2. The National Institute of Neurological Disorders and Stroke (NINDS): NINDS offers comprehensive information on sleep disorders and their impact on health.

  3. Mayo Clinic: Mayo Clinic’s website includes articles and guides on various sleep-related topics, from sleep disorders to tips for better sleep. 

  4. Harvard Health Publishing – Harvard Medical School: Harvard Medical School provides expert insights into the science of sleep, the effects of sleep deprivation, and tips for improving sleep. 

  5. The Sleep Research Society: The Sleep Research Society offers valuable resources and research articles on sleep and circadian rhythms. 

  6. American Academy of Sleep Medicine: AASM provides guidelines, publications, and educational resources related to sleep medicine and sleep disorders. 

  7. Centers for Disease Control and Prevention (CDC): The CDC offers information on sleep and its impact on public health, including sleep-related statistics and resources. 

  8. The Sleep Council (UK): The Sleep Council provides information and tips on sleep hygiene, mattress selection, and creating a sleep-conducive environment. 

  9. WebMD – Sleep Disorders Health Center: WebMD’s Sleep Disorders Health Center includes articles, expert advice, and interactive tools to help readers understand and improve their sleep. 

  10. PubMed: For academic and research-based information on sleep, you can search PubMed, a database of scientific articles and studies related to sleep and sleep disorders. 

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