Skip to content


Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement (REM) sleep,and reduced interactions with surroundings. Distinguished from wakefulness by a decreased ability to react to stimuli.

Most of the content of this article is extracted from the Why we sleep book by Matthew Walker

Consequences of lack of sleep

Sleeping less than six or seven hours a night can produce these consequences:

  • Demolishing of the immune system.
  • Doubling your risk of cancer.
  • Is a key lifestyle factor determining and worsening the development of the Alzheimer's disease.
  • Disruption of blood sugar levels so profoundly that you would be classified as pre-diabetic.
  • Increase the likelihood of block and brittle of your coronary arteries. Setting you on a path toward cardiovascular disease, stroke, and congestive heart failure.
  • Contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.
  • Swelling concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction.
  • Thwart the ability to learn and memorize.

A balanced diet and exercise are of vital importance, but we now see sleep as the key factor in health. The physical and mental impairments caused by one night of bad sleep dwarf those caused by an equivalent absence of food or exercise.

Therefore, the shorter you sleep, the shorter your life span.

Sleep benefits

We sleep for a lot of nighttime benefits that service both our brains and our body. There does not seem to be one major organ within the body, or process within the brain, that isn't optimally enhanced by sleep.

Within the brain, sleep enriches our ability to learn, memorize and make logical decisions and choices. It recalibrates our emotional brain circuits, allowing us to navigate next day social and psychological challenges with cool-headed composture.

Downstairs in the body, sleep:

  • Restocks the armory of our immune system: helping fight malignancy, preventing infection, and warding off sickness.
  • Reforms the body's metabolic state by fine-tuning the balance of insulin and circulating glucose.
  • Regulates our appetite, helping control body weight through healthy food selection rather than rash impulsivity.
  • Maintains a flourishing microbiome within your gut essential to our nutritional health being.
  • Is tied to the fitness of our cardiovascular system, lowering blood pressure while keeping our hearts in fine condition.

Dreaming produces a neurochemical bath that mollifies painful memories and a virtual reality space in which the brain melds past and present knowledge, inspiring creativity.

Therefore, Sleep is the single most effective thing we can do to reset our brain and body health each day.

Sleep physiological effects

There are two main factors that determine when you want to sleep or stay awake:

  • The signal sent by the suprachiasmatic nucleus following the circadian rhythm.
  • Sleep pressure: The brain builds up a chemical substance that creates the "sleep pressure". The longer you've been awake, the more that chemical sleep pressure accumulates, and consequentially, the sleepier you feel.

The circadian rhythm

We have an internal clock deep within the brain, called the suprachiasmatic nucleus, that creates a cycling, day-night rhythm, known as circadian rhythm, that makes you feel tired or alert at regular times of night and day, respectively. The circadian rhythm determines:

  • When you want to be awake or asleep.
  • Your timed preferences for eating and drinking.
  • Your moods and emotions
  • The amount of urine you produce.
  • Your core body temperature.
  • Your metabolic rate.
  • The release of numerous hormones.

Contrary to common belief, circadian rhythm is not defined by the daylight sun cycle. As Kleitman and Richardson demonstrated in 1938:

  • When cut off from the daily cycle of light and dark, the body keeps on maintaining the rhythm.
  • The period of the circadian rhythm is different for each person, but has an average of 24 hours and 15 minutes.

Even if it's not defined by the sun light, it corrects those 15 minutes of delay to stay in sync with it. The suprachiasmatic nucleus can readjust by about one hour each day, that is why jet lag can be spawn through multiple days.

That reset does not come free. Studies in airplane cabin crews who frequently fly on long haul routes and have little chance to recover have registered:

  • The part of the brains related to learning and memory had physically shrunk, suggesting the destruction of brain cells caused by the biological stress of timezone travel.
  • Their short term memory was significantly impaired.
  • They had far higher rates of cancer and type 2 diabetes than the general population.

The peak and valley points of wakefulness or sleepiness vary too between people, it's known as their chronotype and it's strongly determined by genetics. The chronotype defines three types of people:

  • Morning types: They have their peak of wakefulness early in the day and the sleepiness early at night. They prefer to wake at or around dawn, and function optimally at this time of day.
  • Evening types: They prefer going to bed late and subsequently wake up late the following morning, or even in the afternoon.
  • In between: The remaining people fall somewhere in between, with a slight leaning towards eveningness.


The suprachiasmatic nucleus communicates its repeating signal of day and night to your brain and body by releasing melatonin into the bloodstream from the pineal gland. Soon after dusk, the suprachiasmatic nucleus starts increasing the levels of this hormone, telling the rest of the body that it's time to sleep. But melatonin has little influence on the generation of sleep itself.

Once sleep is under way, melatonin decreases in concentration across the night and into the morning hours. With dawn, as sunlight enters the brain through the eyes (even through the closed lids), the pineal gland is instructed to stop releasing melatonin. The absence of circulating melatonin now informs the brain and body that it's time to return to a wakefulness active state for the rest of the day

Sleep pressure

While you are awake, the brain is releasing a chemical called adenosine. One consequence of the increasing accumulation of adenosine is the increase of the desire to sleep by turning down the "volume" of wake promoting regions in the brain and turning up the sleep inducing ones. Most people fall to the pressure after twelve to sixteen hours of being awake.


You can artificially mute the sleep signal of adenosine by using a chemical that makes you feel more alert and awake, such as caffeine. Caffeine works by battling with adenosine for the privilege of latching on to adenosine receptors in the brain. Once caffeine occupies these receptors, it does not stimulate them like adenosine, making you sleepy. Rather, caffeine blocks and effectively inactivates the receptors acting as a masking agent.

Levels of caffeine peak around thirty minutes after ingestion. What is problematic, though, is the persistence of caffeine in your system. It takes between five to seven hours to remove 50 percent of the caffeine concentration from your body.

An enzyme within your liver removes caffeine from your system. Based in large part on genetics, some people have a more efficient version of the enzyme that degrades caffeine, allowing the liver to clear it from the bloodstream. Age is also a variable to take into account, the older we are the longer it takes our brain and body to remove it.

When your liver evicts the caffeine from your system, you encounter the caffeine crash. Your energy levels plummet rapidly, finding difficult to function and concentrate, with a strong sense of sleepiness once again.

For the entire time that caffeine is in your system, the adenosine keeps on building up. Your brain is not aware of this rising tide of sleep encouraging chemical because the wall of caffeine is holding it back from your perception. Once your liver dismantles the barricade, you feel a vicious backlash: you are hit with the sleepiness you had experienced two or three hours ago before you drank that cup of coffee plus all the extra adenosine that has accumulated in the hours in between.

Relationship between the circadian rhythm and the sleep pressure

The two governing forces that regulate your sleep are ignorant of each other. Although they are not coupled, they are usually aligned.

Starting on the far left of the figure, the circadian rhythm begins to increase its activity a few hours before you wake up. It infuses the brain and body with an alerting energy signal. At first, the signal is faint, but gradually it builds with time. By early afternoon, the activating signal from the circadian rhythm peaks.

Now let's look at the sleep pressure pattern. By mid to late morning, you have only been awake for a half of hours. As a result, adenosine concentrations have increased a little. Furthermore, the circadian rhythm is on its powerful upswing of alertness. This combination of strong activating output from the circadian rhythm together with low levels of adenosine result in a delightful sensation of being wide awake.

The distance between the curved lines above will be a direct reflection of your desire to sleep.

By eleven pm, you've been awake for fifteen hours, and your brain is drenched in high concentrations of adenosine. Additionally, the circadian rhythm line is descending, powering down your activity and alertness levels. This powerful combination triggers a strong desire for sleep.

During sleep, a mass evacuation of adenosine gets under way as the brain has the chance to degrade and remove it. After eight hours of healthy sleep, the adenosine purge is complete. As this process is ending, the circadian activity rhythm has returned, and its energizing influence starts to approach, therefore naturally waking us up.


Scientists can demonstrate that the two forces determining when you want to be awake and sleep are independent and can be decoupled from their normal lockstep.

When you skip one night's sleep and remain awake throughout the following day,

By remaining awake, and blocking access to the adenosine drain that sleep opens up, the brain is unable to rid itself of the chemical sleep pressure. The mounting adenosine levels continue to rise. This should mean that the longer you are awake, the sleepier you feel. But that's not true. Though you will feel increasingly sleepy throughout the nighttime phase, hitting a low point in your alertness around five or six in the morning, thereafter, you'll start to be more awake. This effect is answered by the energy return of the circadian rhythm. Unlike sleep pressure, the circadian rhythm pays no attention to whether you are asleep or awake.

Am I getting enough sleep?

When you don't sleep enough, one consequence among many is that adenosine concentrations remain too high, so the next morning you continue to accumulate sleep debt

If after waking up you could fall asleep at ten or eleven in the morning, it means that you're likely not getting enough sleep quantity or quality. The same can be said if you can't function optimally without caffeine before noon, you'll be most likely self-medicating your state of chronic sleep deprivation.

Other sleep indicators can be if you would sleep more if you didn't set an alarm clock, or if you find yourself at your computer screen reading and then rereading the same sentence.

Of course, even if you are giving yourself plenty of time to get a full night of shut-eye, next-day fatigue and sleepiness can still occur because you are suffering from an undiagnosed sleep disorder.

The sleep cycle

Humans cycle through two types of sleep in a regular pattern throughout the night with a period of 90 minutes. They were called non-rapid eye movement (NREM) and rapid eye movement (REM). Later, NREM sleep was further subdivided into four separate stages, named from 1 to 4 (all awful names (҂⌣̀_⌣́)). Stages 3 and 4 are the deepest stages of NREM sleep, meaning that it's more difficult to wake you up in comparison with stages 1 and 2.

In REM sleep, your eyes rapidly move from side to side underneath the lids. This movement are accompanied by active brainwaves, almost identical to those observed when you are awake. On the other hand, eyes remain still and the brainwaves also calm down in the NREM phases.

Even though we switch from sleep phases each 90 minutes, the ratio of NREM to REM sleep throughout the night changes across the night. In the first half of the night, the vast majority of time is spent in deep NREM and very little REM. But as we transition through the second half of the night, REM starts dominating.

Although there is no scientific consensus, the need to remodel and update our neural circuits at night can explain this repeatable but asymmetric pattern. Throughout the day, the new memories are stored in the RAM of your brain, when you start to sleep, the brain needs to move the important ones to the hard drive, for long term retrieval. The brain needs to solve an optimization problem:

  • The hard drive and the RAM have limited capacity.
  • The RAM needs to be cleaned to be able to register the next day's memories.
  • The brain needs RAM to do the analysis of which memories to keep and which to remove.

A key function of NREM sleep is to remove unnecessary neural connections, while REM sleep plays a role in strengthening those connections. The different roles and the capacity limits explains why the brain needs to switch between them. The asymmetry can be explained with the simile of creating a sculpture from a block of clay. At the beginning of the night, the long phases of NREM extensively removes unneeded material, with short REM phases to define the basic form. With each cycle, less material needs to be strongly removed and more enhancing of the details is required, thus the increase of REM sleep.

A danger resides in this sleep profile. Since your brain desires most of its REM sleep in the last part of the night, if you wake up early, sleeping 6 hours instead of 8, you can be losing between 60 to 90% of all your REM sleep, even though you are losing 25% of your total sleep time. It works both ways, if you instead go to sleep two hours late, you'll loose a significant amount of deep NREM sleep. Preventing the brain to have the required REM or NREM daily rations results in many physical and mental issues.

Sleeping time and sense distortions

When you're asleep, you loose awareness of the outside world. Your ears are still hearing, your eyes, though closed, are still seeing, and the rest of the organs keep on working too.

All these signals still flood into the center of your brain, but it's there, in the sensory convergence zone, where they end. The thalamus is the sensory gate to the brain that blocks them. If it lets them pass, they travel to the cortex at the top of your brain, where they are consciously perceived. By locking its gates shut when you're asleep, the thalamus imposes a sensory blackout in the brain. As a result, you are no longer consciously aware of the information transmitted from your sense organs.

Another consequence of sleeping is a sense of time distortion experienced in two contradictory ways. While you loose your conscious mapping during sleep, at a non-conscious level, the brain keeps track of time with incredible precision. To distort it even more, you sense a time dilation in dreams. The signature patterns of brain-cell activity that occurs as you learn, gets recurrently repeated during sleep. That is, memories are being replayed at the level of brain-cell activity as you sleep. During REM sleep, the memories are replayed at half or quarter the speed in comparison of the activity when you're awake. This slow neural recounting may be the reason why we have that time dilation.

How your brain generates sleep

Brainwave activity of REM sleep looks similar to the one you have when you're awake. They cycle (going up and down) at a fast frequency of thirty or forty times per second in an unreliable pattern. This behaviour is explained by the fact that different parts of your waking brain are processing different pieces of information at different moments in time and in different ways.