Why We Sleep


Dr Matthew Walker, Ph.D, has spent most of his career to study sleep.

This is a summary of his book “ Why We Sleep: The New Science of Sleep and Dreams”.

Sleep is universal in animals (even in insects and worms) and is a vital function.

How Sleep Works

Sleep Rhythm

There are 2 mechanisms that regulate sleep: the circadian rhythm and adenosine.

Circadian rhythm- regulated by Melatonin produced by the suprachiasmatic nucleus in the brain. It is the natural wake drive which responds to light and darkness and melatonin signals to the brain that it is dark but it has little influence on the generation of sleep itself. When sleep is underway, melatonin slowly decreases in concentration across the night and with dawn, sunlight puts a brake on the pineal gland to release melatonin.

Adenosine- a chemical that causes the increased desire to sleep. It rises consistently throughout the day when you are awake without sleep. Sleep naturally happens when your adenosine is at its highest and your circadian “wake drive” is at its lowest.During sleep, the brain has the chance to degrade and remove the day’s adenosine.

The Human Sleep Cycle

The brain switches between 2 types of sleep- REM (Rapid Eye Movement) and nonREM sleep.
-NREM clears out old memories and mental “trash” and moves information into long term storage.
- REM strengthens the valuable information that remains, and it forges novel connections between them.
When you sleep, the brain goes through sleep cycles that each lasts about 90 minutes. Each sleep cycle generally begins with NREM sleep then ends with REM sleep. As one cycle ends, the next begins. The ratio of NREM sleep to REM sleep within the 90 minutes cycle changes dramatically across the night. In the first half of the night, the vast majority of the 90 min are consumed by deep NREM sleep and very little REM sleep. As you transition to the second half of the night, most of the time is dominated by REM sleep with little, if any, deep REM sleep.

The Brainwaves of Wake and Sleep:
Prior to bed, your waking brain activity is frenetic with fast frequency brain activity andin erratic ways. The electrical chaos 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.
NREM sleep has 4 stages: light NREM stages 1 and 2, and deep NREM in stages 3 and 4. The brainwave activityrapidly decelerates perhaps to 2 to 4 waves per second in deep sleep (slow wave sleep).
It can be considered that the waking brainwave activity as principally concerned with the receptionof the outside sensory world while the state of deep NREM slow wave sleep is a state of inward reflection to foster information transfer and the distillation of memories. The brainwaves of REM sleep and wake are so similar but during REM sleep your muscles are completely paralyzed.

Sleep improves long term factual recall:
Your brain stores different memories in different places. The hippocampus stores short term memory with a limited capacity. The cortex stores long term memory in large storage bank.

The slow wave, NREM sleep moves facts from the hippocampus to the cortex. This has 2 positive effects: it secures memories for the long-term and it clears out short term memory to make room for new information, improving future learning.

2) Sleep has memory-boosting benefits:

Motor skill memories: sleep helps the brain automate the movement routine, making them second nature.
It is related to the amount of stage 2NREM, especially the last 2 hours of an eight-hour night of sleep.The region above the motor cortex (just in front of the crown of the head) is responsible for this offline memory boost.
Recovery of motor function in stroke patients is due in part to the hard night by night work of sleep. Following a stroke, the brain begins to reconfigure those neural connections that remain,and sprout new connections around the damaged zone. There is preliminary evidence that sleep is one critical ingredient assisting in this neural recovery effort.
3) Sleep for creativity: sleep provides a night-time theater in which the brain tests out and builds connections between vast stores of information. It happens in the dreaming state- REM sleep.
B- Sleep Deprivation

Sleep deprivation is harmful to the brain. It worsens attention and concentration, worsens emotion control and contributes to numerous neurological and psychiatric conditions such as Alzheimer’s disease, anxiety, depression, bipolar disorder, suicide, stroke, chronic pain.
Sleep deprivation disrupts the normal function of many physiological processes, likely contributing to chronic diseases such as cancer, diabetes, heart attacks, weight gain, obesity and immune deficiency.
One or two hours of sleep deprivation triggers the sympathetic nervous system (fight or flight response) and disrupts hormonal balances.
Lack of sleep can cause a momentary lapse in concentration called microsleep. It can cause accident if you are driving.
Partial sleep deprivation can cause performance impairment but the person may not be aware of it. With chronic sleep restriction over months or years, an individual will actually acclimate to their impaired performance, lower alertness and reduced energy levels.
Chronic lack of sleep incompetitive athletes is related to a markedly higher risk of injury.
Power naps may momentarily increase basic concentration under conditions of sleep deprivation as can caffeine up to a certain dose. But researchers found that neither naps or caffeine can salvage more complex functions of the brain, including learning, memory, emotional stability, complex reasoning, or decision making.
It is very rare that a person can have a rare gene and who could survive on 6 hours of sleep and show minimal impairment. The odd of being struck by lightning (1 in 12,000) is higher than having that rare gene.
Many students do not sleep to cram for an exam. Studies showed that learning-related activity in the hippocampus are much reduced in the sleep deprived individuals. The few memories that you are able to learn while sleep deprived are forgotten far more quickly in the hours and days thereafter. Memories formed without sleep are weaker memories, evaporating rapidly. The crucial role for sleep is to consolidate recently learned memories.

Sleep and Alzheimer’s: there is a connection between insufficient sleep especially the deep NREM sleep. During sleep the glial cells of the brain were shrinking in size by up to 60 % during NREM sleep , enlarging the space around the neurons and allowing the cerebrospinal fluidto proficiently clean out the metabolic refuse left by the day’s neural activity. One toxic debris is amyloid protein associated with Alzheimer’s. Without sufficient sleep, amyloid plaques build up in the brain, especially in deep -sleep-generating regions.

Sleep loss and the immune system:
Sleep fights against infections and sickness. Study of participants who had 8 hours sleep in the week before getting the flu injection generated a powerful antibody reaction comparing to the sleep deprived group. A brief dose of short sleep can affect the cancer-fighting immune cells. A lack of sleep will encourage cancer development and spreading (metastasis) in mice. WHO ( World Health Organization ) has officially classified night-time shift work as a “probable carcinogen”.

C- Dreams

Dreaming is a bizarre experience. You are unconscious but you can perceive intense vivid sensations and hallucinate things that aren’t there. You feel like you are moving in the world but your muscles are in a state of paralysis. You remember faces and memories that you haven’t thought about for years, maybe decades. You have no control over your emotions, swinging from intense rage and jealousy to exuberance. Finally, when you wake up, you promptly forgot everything. If you experienced all this while awake, you would think you were having a psychotic episode.

Freud considered dreams as expressions of repressed desires as he built a psychological movement around interpreting dreams as such but the interpretation methodswere so subjective and cannot be proven.

During deep NREM sleep, overall metabolic activity shows a modest decrease comparing to the awake state. In REM sleep, numerous parts of the brain “light up” on the MRI scan. REM sleep is characterized by strong activation in visual, motor, emotional and autobiographical memory regions of the brain and a relative deactivation in regions that control rational thought.

During REM sleep, the brain is devoid of Noradrenaline, an anxiety triggering molecule.

Sleep and specifically REM sleep was clearly needed for us to heal emotional wounds.In a study, Cartwright demonstrated that patients who were expressly dreaming about the painful experiences around the time of the events who went on to gain clinical resolution from their despair, mentally recovering a year later.

In PTSD patients, there is excessively high levels of noradrenaline in the brain during REM sleep dreaming so the brain at night cannot strip away the emotion from the trauma memory, since the stress chemical is too high. Dr Murray Raskind found that Prazosin can alleviate the reoccurring nightmares in his PTSD patients. Prazosin suppresses noradrenaline in the brain.

REM sleep helps with accurately reading expressions and emotions of faces. When sleep deprived, people could no longer distinguish one emotion from another with accuracy and they slipped into a default of fear bias believing even gentle or somewhat friendly looking faces to be menacing.

Dream creativity:

REM sleep and dreaming have another distinct benefit: intelligent information processing that inspires creativity and promotes problem solving.

D- Sleep Disorders

Somnambulism (Sleepwalking): It is the act of walking and performing other behaviour while asleep. It happens during NREM sleep and not in REM dreaming sleep.

Neurologically, sleepwalking is accompanied by an unexpected spike in nervous system activity, causing the person to be stuck somewhere between sleep and wakefulness.

Insomnia: people suffer from an inadequate ability to generate good sleep quality or quantity despite having the adequate opportunity to get sleep. Some people have paradoxical insomnia having an illusion or misperception of poor sleep that is not actually poor. There are 2 subsets: sleep onset insomnia and sleep maintenance insomnia. Clinical insomnia is defined as insomnia for more than 3 nights a week and for more than 3 months. The 2 most common triggers of chromic insomnia are psychological distress such as emotional concerns or anxiety.The sympathetic nervous system is overactive. The emotion -generating regions (the amygdala) and memory recollection centre (hippocampus) all remained active. The thalamus- the sensory gate of the brain that needs to close shut to allow sleep, remained active. The insomnia patients could not disengage from the worrisome, ruminative brain activity. Patients also have a lower quality sleep, less powerful brainwaves during deep NREM and have more fragmented REM sleep so they wake up not feeling refreshed and unable to function well during the day cognitively and/ or emotionally.


There are 3 core symptoms: excessive daytime sleepiness, sleep paralysis and cataplexy (sudden loss of muscle control). The sleep-wake switch in the hypothalamus releases a neurotransmitter called orexin. When orexin is released, the brain stem pushes open the sensory gate of the thalamus, allowing the perceptual world to flood in the brain transitioning to wakefulness. At night, the sleep-wake switch stops releasing orexin. The power station of the brainstem is close and we lost the perception with the outside world. Narcoleptic patients have low levels of orexin and the switch endlessly flickers on and off. Suvorexant (Belsomra) blocks orexin at night but people slept only 6 min earlier than placebo.

Fatal Familial insomnia:it is caused by a genetic defect

Total Sleep deprivation :Death ensued as quickly from total sleep deprivation as it did from total food deprivation in a study in rats.

E- Factors affecting Sleep

Ipads, blue LED lights

Reading an Ipad for several hours before bed suppressed melatonin by over 50 percent at night and delayed the rise of melatonin by up to 3 hours relative to the natural rise in these same individuals when reading a printed book.Individuals lost significant amounts of REM sleep following Ipad reading. Secondly, they also felt less rested and sleepier throughout the day and thirdly there was a lingering after-effect with a 90 minutes lag in their evening melatonin rising for several days after Ipad use ceased like a digital hangover effect.
Caffeine and sleep:
Adenosine is building up in the brain with each waking minute and it causes a sleep pressure, that is an increasing desire to sleep. Caffeine is competing with adenosine to bind on the adenosine receptors in the brain and effectively inactivates the receptors blocking the sleepiness signal. Caffeine has a half- life of 5 to 7 hours and is removed by an enzyme from the liver. When the liver successfully removes all the caffeine, there is a “caffeine crash” and you feel a strong sense of sleepiness again. Unless you consume more caffeine to act against adenosine, you will find it very difficult to stay awake. You would then start a dependency cycle.
Alcohol is in the class of drugs as sedative.In moderate doses, alcohol increased sociability by sedating the frontal lobe, the part of brain that control our impulses and restraints our behaviour. As a result, people “loosen up” becoming more social. Give a little more time, it begins to sedate other parts of the brain and dragging down into a stupefied state. Alcohol sedates people out of wakefulness but not induce a natural sleep. The brainwave state is not of sleep but a light form of anaesthesia. Alcohol harms sleep in 2 ways. First alcohol fragments sleep with brief awakenings. Therefore, sleep is not restorative. Unfortunately, most of the awakenings go unnoticedby the sleeper and to link alcohol consumption with feelings of next day exhaustion. Second, alcohol will often suppress REM sleep. In alcoholics who went with long stretches of time without dream sleep, the REM sleep pressure can erupt into waking consciousness causing hallucinations, delusions and gross disorientation. This process contributes to the psychotic state in alcoholics called “delirium tremens”.
The overnight work of REM sleep, which normally assimilates complex memory knowledge, had been interfered with by alcohol. More surprisingly, the brain is not done processing learning knowledge after the first night sleep. Memories remain vulnerable to disruption of sleep (including that from alcohol) even up to 3 nights after learning, despite 2 full nights of natural sleep prior.

Get the nighttime chills:
The ambient room temperature can affect the quality of sleep. It is easier to fall asleep in a room that is too cold than too hot. The nocturnal melatonin levels are controlled by the loss of daylight at dusk , but also the drop in temperature that coincides with the setting sun. We sleep more quickly when the core temperature is reduced.

Sleeping pills:
Sleeping pills, old and new, target the same system in the brain that alcohol does- they are sedatives and knock out the prefrontal cortex. The unwanted side-effects include next day grogginess, day-time forgetfulness, slowed reaction times during the day that can impact motor skills such as driving. The waking grogginess can lead to people to take more cups of coffee or tea throughout the day worsening the insomnia and then taking a sleeping pill at night, thus perpetuating a possible downward spiral. The other feature of sleeping pills is rebound insomnia when the pills are stopped.

Kripke and his colleagues found that people taking sleeping pills were 4.6 times more likely to die over a two-and-a half-year period than those who were not using sleeping pills. The heavy users are defined as taking more than 132 pills a year than those not taking sleeping pills. Even occasional users taking just 18 pills per year were still 3.6 times more likely to die at some point than non-users.The causes of the increase of death were unclear.

Sleep Hygiene:
The obvious methods involve reducing caffeine and alcohol intake, removing screen technology from the bedroom and having a cool bedroom.
         ●    Establish a regular bedtime and wake-up time, even on weekends.

         ●    Go to bed only when sleepy and avoid sleeping on the couch early/mid evenings

         ●    Never lie in bed for a significant time period, rather get out of bed and do something quiet and relaxinguntil the urge to sleep returns

         ●    Avoid daytime nappingafter 3pm if you are having difficulty sleeping at night

         ●    Reduce anxiety provoking thoughts and worries by learning to mentally decelerate before bed

         ●    Remove visible clock faces from view in the bedroom, preventing clock watching anxiety at night.

         ●    Restrict the time spent in bed perhaps even to just 6 hours of sleep or less to begin with. By keeping patients awake longer, we build up a strong sleep pressure- a greater abundance of adenosine. The patients then fall asleep faster and achieve a more stable form of sleep. Upon reestablishing a patient’s confidence, time in bed is gradually increased.

         ●    There is a bidirectional relationship between sleep and exercise. Try to exercise at least 30 minutes on most days but not to exercise just before bed, better at least a few hours before as physical exertion can increase the core temperature. Sleep, in return, will boost your fitness and energy, setting in motion a positive, self- sustaining cycle of improved physical activity and mental health.

         ●    Avoid large meals and beverages late at night. A light snack is okey but a large meal can cause indigestion which interferes with sleep. Drinking too many fluids at night can cause frequent awakenings to urinate.

         ●    Take a hot bath before bed. The drop in body temperature after getting out of bath may help you feel sleepy, and the bath can help you to relax and slow down.

Try to get outside in natural sunlight for at least thirty minutes each day. If possible, wake up with the sun or use very bright lights in the morning and turn down the lights before bedtime.