Why We Sleep?
Have you ever wondered why humans sleep? And what the benefits may be, if any? Have you ever been curious as to why you feel sleepy and awake at regular times of the night and day? As if you are being controlled by an internal clock? Have you ever met someone whose sleep-wake cycle seems almost opposite to your own? And you wonder how or why this is? Have you ever wondered how caffeine works? The most widely used psychoactive substance in the world. And one of the leading causes of insomnia. Have you ever heard of the different stages of sleep? Non-REM sleep (deep sleep) and REM (dream sleep). Have you ever wondered why we dream? Or if any other animals on Earth share our same twilight fantasies?
Well, the most fascinating book that I have yet to read. Written by Mathew Walker. A professor of neuroscience and psychology. And the director of the Center for Human Sleep Science at the University of California, Berkley. Provides more answers on sleep and dreaming than I ever thought scientifically possible. This is a preview of part 1/4. I have rewritten some of my favourites paragraphs from his book. Although a heavy read. It will absolutely transform your health. And appreciation for satisfying sleep and meaningful dreaming.
Don't worry if you happen to fall asleep while reading. In fact, Matthew would encourage you to.
Functions of Sleep:
Within the Brain:
Enhances our learning, memory and decision making.
Enhances our ability to connect and socialise with others.
Improves our psychological health.
Keeps us cool, calm and composed.
Within the Body:
Boosts our immunity.
Improves our blood sugar balance.
Improves our ability to make healthy food choices.
Improves our microbiome (the root of human health).
Improves our cardiovascular health (blood pressure and heart).
*Sleep is the single most effective thing we can do daily to reset our brain and body.
What controls our Sleep-Wake-Cycle?
24-Hour Clock (Circadian Rhythm):
Through evolutionary adaptations, humans have developed a biological clock. This sleep-wake cycle causes us to feel tired or alert at regular times of the night and day.
A substance known as adenosine, builds up in your brain, creating a pressure and desire to sleep. The longer we remain awake - the more adenosine that accumulates - the more tired we become.
The balance between our 24-hour clock (circadian rhythm) & sleep pressure. Is what dictates our alertness during the day and our ability to sleep at night.
The circadian rhythm:
Every living creature on the planet generates this natural sleep cycle. It was in 1729 when we first discovered that plants generate their own internal time. If you place a plant in a sealed box for 24 hours. Putting the plant in total darkness for both day and night. Despite being cut off from the influence of light during the day. The plant will still behave as though it were being bathed in sunlight. Its leaves will expand proudly. It will then retract its leaves as if on cue at the end of the day. Even without the suns setting signal. The leaves will stay collapsed throughout the entire night. This proves that somewhere within the plant is a 24-hour rhythm generator. That could track time without any cues from the outside world, such as daylight.
Humans also generate their own circadian rhythm. In the absence of external light from the sun. Even individuals with blindness do not lose their circadian rhythm. Daylight is the most reliable, repeating signal that we have in our environment. Since the birth of our planet and every single day thereafter. Without fail. The sun has always risen in the morning and set in the evening. Thus, most living species likely adopted a circadian rhythm. To synchronize themselves with the daily orbital of planet Earth.
Functions of the Circadian Rhythm:
Controls our eating and drinking habits.
Regulates our mood and emotions.
Controls urine output.
Controls our core temperature.
Controls our metabolic rate.
Controls the release of many hormones.
Our 24-hour biological clock sits right in the middle of our brain. And is called the suprachiasmatic nucleus. The suprachiasmatic nucleus contains 20 000 brain cells. It uses reliable light information to reset its accuracy to a spot-on 24-hour cycle. Preventing any deviation.
Variability in Circadian Cycles:
Although every human being displays an unyielding 24-hour pattern. The peak and tough points are strikingly different from one individual to another.
For some people (about 40% of the population), according to their circadian rhythm. Their peak wakefulness is early in the day and their peak sleepiness is early at night. They prefer to wake up around dawn and can function optimally at this time of day.
For some people (about 30% of the population), their peak wakefulness is later in the day and their peak sleepiness is later at night. They naturally prefer to go to bed late and wake up late morning/ early afternoon. These people are incapable of falling asleep early at night no matter how hard they try. They strongly dislike waking up early. If they do, their brain (specifically the prefrontal cortex) remains in a more sleep-like state. They cannot function efficiently early morning. Night owls are not owls by choice. They are bound to a delayed schedule by unavoidable DNA hardwiring.
The remaining 30% of people lie somewhere in between morning and evening types. With a slight leaning towards eveningness.
Why the Variation?
You must be wondering why mother nature would program this variability across people. Well, people likely evolved to co-sleep as families or even whole tribes, not alone or as a couple. The night owls in the group would not be going to sleep until one or two am and not waking until nine or ten am. The morning larks would have retired for the night at nine pm and woken up at five am. Consequently, the group as a whole is only vulnerable (every person asleep) for just four hours. Instead of eight hours (despite everyone still getting a full night's rest of eight hours). That is a 50% increase in survival fitness. Hence the usefulness of variability in circadian cycles.
Your suprachiasmatic nucleus communicates the repeating signal of day and night to your brain and body. It does this by using a messenger hormone known as melatonin or “the vampire hormone”. The rise in melatonin begins shortly after dusk and helps to regulate the timing of when sleep occurs. It has little influence on the generation of sleep. Melatonin provides the official instruction to commence the event of sleep but does not participate in sleep itself. Once sleep is underway, melatonin slowly starts decreasing in concentration across the night and into the morning hours. With dawn, as sunlight enters the brain through the eyes (even through closed eyelids), melatonin production is inhibited. This informs the brain and body that full sleep has been reached, in this regard human beings are “solar-powered”.
Sleep Pressure (Adenosine):
The substance, Adenosine, produced within the brain. Begins to increase in concentration with every waking minute that elapses. The longer you are awake the more adenosine that will accumulate. The consequence of increasing adenosine in the brain is an increasing desire to sleep. This is known as sleep pressure. When adenosine reaches peak concentrations, an irresistible urge for sleep will take hold. This happens to most people after 12 – 16 hours of being awake.
One way to artificially suppress sleepiness and prolong alertness is by consuming caffeine. Caffeine occupies the same receptor sites in the brain as adenosine, blocking its action. Therefore, caffeine doesn’t necessarily enhance our wakefulness. But instead, it maintains it while suppressing our desire to sleep. Caffeine tricks you into feeling alert and awake. Despite the high levels of adenosine that otherwise sedates you to sleep. A single dose of caffeine (1 cup). Reaches peak concentration 30 minutes after oral administration. The problem lies in the persistence of caffeine in your system. Caffeine has a half-life of 5-7 hours. Meaning that it takes 5-7 hours for your body to remove 50% of the caffeine from your system. And only after 10-14 hours will your body be completely caffeine-free. Therefore, if you have your first coffee at 8 am. Only between 6-10 pm will your body finally be caffeine-free. This is why consuming caffeine can greatly impair your quality of sleep. Even as little as 1 cup a day early morning. Decaffeinated coffee still contains 15-30 % of the dose of a regular cup of coffee.
Caffeine is removed from your system by an enzyme within your liver. Caffeine gradually degrades this enzyme over time. Based in large on genetics. Some people have a more efficient version of the enzyme that breaks down caffeine. Allowing the liver to clear caffeine much more rapidly from your system. Others have a slower-acting version of the enzyme. It takes far longer for their system to eliminate the same amount of caffeine. Ageing also alters the speed of caffeine clearance. The older we are the longer it takes our brain and body to remove caffeine. And thus the more sensitive we become in later life to caffeine's sleep-disrupting influence.
For the entire time that caffeine is in your system, adenosine nevertheless continues to build up in the background as time passes. Once your liver removes the last remaining caffeine from your system. The accumulated adenosine can bind to its now vacant receptor sites. Which floods your brain with a forceful need to sleep. Initiating the all too common caffeine crash.
What Happens to all the Accumulated Adenosine Once you Sleep?
During sleep, the brain starts to breakdown and remove the adenosine that has accumulated throughout the day. After approximately 8 hours of sleep. Adenosine is cleared entirely, lifting the heavyweight of sleep pressure.
When you do not get enough sleep (8-9 hours). One consequence among many is that adenosine concentrations remain too high. Come the morning, some of yesterday's adenosine will remain. You then carry that outstanding sleepiness balance throughout the following day. This sleep debt will continue to accumulate day after day. Producing a condition of chronic sleep deprivation. This accumulation of adenosine (sleep debt), due to a lack of sufficient sleep. Results in a feeling of chronic fatigue. Manifesting in many forms of mental and physical ailments. That is rife throughout industrialised nations.
NREM and REM Sleep:
Humans don’t just sleep but cycle through two completely different types of sleep. Namely, NREM (non-rapid eye movement) and REM (rapid eye movement).
REM, in which brain activity is almost identical to that when we are awake. Is intimately connected to the experience we call dreaming and is therefore also known as “dream sleep”.
NREM, also known as “deep sleep”. Is divided into four stages, which progressively increases in-depth as we move from stage 1 to 4.
The first half of our 8 hours of sleep is dominated by NREM and the second half is dominated by REM sleep. It is not only humans that sleep in this repeatable but dramatically asymmetric pattern. So do all other mammals and birds.
What are the Functions of NREM and REM Sleep?
NREM, removes unnecessary neural connections, whereas REM strengthens those connections. Our brain always requires a new bout of sleep and its varied stages each night. To auto-update our memory networks based on the events of the prior day.
The Importance of 8 Hours of Sleep:
When you sleep only 6 hours, losing the last two hours of your 8-hour sleep cycle. You will lose 60-90% of your REM sleep (dream sleep). As most of your REM dominates in the second half of your 8-hour sleep cycle. Conversely, when you only sleep 6 hours, losing the first two hours of your 8-hour sleep cycle. You will lose 60-90% of your NREM sleep (deep sleep). As most of your NREM sleep dominates in the first half of your 8-hour sleep cycle.
What is Sleep Paralysis?
During REM sleep, your voluntary muscles are completely paralyzed. However your involuntary muscles those which control your heartbeat and breathing. Operate and maintain life during sleep. This paralysation is known as atonia. It is instigated by a powerful disabling signal that is transmitted down the full length of your spinal cord from your brain stem. Once put in place, the postural body muscles. Such as the biceps of your arms and quadriceps of your legs, lose all tension and strength.
Why did evolution decide to outlaw muscle activity during REM sleep? Because by eliminating muscle activity you are prevented from acting out your dream experience. The brain paralyzes the body so the mind can dream safely.
Without exception, every animal species studied to date sleeps. Or engages in something remarkably like it. This includes insects, such as flies, bees, cockroaches and scorpions. Fish from small perch to the largest sharks. Amphibians such as frogs, and reptiles such as turtles. Komodo dragons and chameleons. All have bona fide sleep. Ascend the evolutionary ladder further and we find that all types of birds and mammals sleep. From shrews to parrots, kangaroos, polar bears, bats and of course humans. Sleep is universal. Even invertebrates, such as primordial molluscs and echinoderms. Even very primitive worms enjoy periods of slumber.