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Why do we sleep? - Capitahealth

Why do we sleep?


Sleep is a recurrent physiologic and fundamental process that every human being experiences, and it provides reparative and restorative bodily functions. The consensus is that sleep is needed to allow us to rest and recover from the stresses of everyday life. Additionally, researchers have shown that a good sleep is essential to maintaining mood, memory, cognitive performance and boosting body immune systems. During the waking hours, a chemical called adenosine builds up in the bloodstream eventually binds with brain cells (specifically in the basal forebrain) and inhibits their activities, which causes drowsiness. When we sleep the level of adenosine reduces, the brain cells are rejuvenated and are active again. Also, a recent study states that some tiny cavities are open up after about 6 hours of sleep, and they facilitates the drainage of some waste products from the brain.

Sleep has different phases; a deep slumber and one that is close to being awake. These phases are associated with changes in human body and health.

Sleep has two categories

  1. NREM
  2. REM

NREM or nor-rapid eye movement sleep has features like

  • Slow breathing and heart rate
  • Reduced physiological activity
  • Slower brain waves
  • Reduced blood pressure

It consists of four stages.

  1. In stage 1, transition from being awake to falling asleep takes place. Nervous system begins to slow down; muscle jerks and falling sensation are common.
  2. In stage 2, the brain waves further slow down with few rapid brain waves called sleep spindles. The heart rate and body temperature decrease. Eye movement stop and light sleep begins.
  3. In stage 3 and 4, the body goes into complete relaxation with a deeper sleep, reduced heart rate, blood pressure and body temperature. Slow brain waves (i.e. Delta waves) are present at these stages, which together refer to ‘slow wave sleep’. If a person wakes up in this phase, he feels disoriented for a while.

REM or rapid eye movement sleep is the sleep in which dreams occur. It has features like

  1. Intense brain activity
  2. Rapid and irregular breathing
  3. Continuous eye movements
  4. Temporary paralyzing of limb muscles
  5. Increased heart rate and blood pressure

A balance between NREM and REM sleep is necessary for a proper full-night sleep and promoting processes like memory, concentration and mood.

Sleep Architecture:

Research shows that REM and NREM sleep occur in alternate cycles of 90-110 minutes with 4-6 repetitions throughout the night.sleeping-child-812181_960_720

For infants, 50% of sleep time is in REM sleep. This figure is 20-25% for normal adults. With advancing age, stages 3 & 4 of NREM sleep decrease and REM dominates. The need of sleep is there, it’s only REM replacing NREM sleep stages in old age.

On an average, the adult sleep has 50% NREM stage 2 sleep, 20% REM sleep and the remaining in other stages; though this time is not constant over a night. At night, the first few cycles are short period REM sleep and longer periods of slow-wave sleep. As the night passes, REM increases and NREM stages decrease with REM sleep dominating by the time of dawn.

Adults need average 7 hours, a teenager about 9.5 hours and an infants 16 hours sleep per day. This pattern of sleep refers to ‘sleep architecture’.

Why sleep is important?

Everyone needs sleep for survival. According to experts, sleep triggers body’s repair process, production of proteins, biochemical and physiological processes.

Sleep loss directly affects functions like memory, reflex actions, complex thoughts and concentration. In addition to making us feel grumpy and groggy, sleep loss may harm endocrine and immune system; it may lead to chronic diseases like obesity, diabetes and hypertension.

Impact of sleep on performance and mood:

Studies show that less than seven hours of sleep a night for a week can seriously impair alertness. People, who are up for 19 hours, score badly in alertness tests. Thus, a sustained sleep deprivation is harmful to memory and motor performance.

Sleep loss adversely affects mood. A research of Pennsylvania University made people sleep for only 4.5 hours a night for a week. The result was a group of angry, sad, exhausted and mentally stressed people. Their mood scores declined as the test progressed. When they got proper sleep at night, there was a dramatic change in their mood scores, thus proving that sleep loss can influence mood.

Hormones and Metabolism

Sleep is immensely important for the body as it’s the time when hormones get secreted. For instance

  • Cortisol, a hormone promoting wakefulness
  • Growth hormone, contributing to childhood growth and muscle mass in adults
  • Follicle stimulating hormone and luteinizing hormone; both involved in reproduction
  • Hormones influencing appetite and weight

Obesity and Diabetes

Obesity is on a rise in America. According to the Centers for Disease Control and Prevention, 65% of Americans now are overweight. In addition to calorie intake, sleep deprivation has been linked to obesity.

  • Sleep deprivation means reduced production of growth hormone. As growth hormone controls the fat and muscle proportions in human body, a reduction in it means obesity.
  • A study correlates inadequate sleep and lesser levels of hormone Leptin needed for regulating the carbohydrate metabolism in body. This in turn causes the body to crave for carbohydrates despite the presence of already consumed fat.

1999 study at University of Chicago links sleep deprivation to impairment of sugar metabolism and hormone levels. Eleven young adults had little sleep for several nights resulting in a decline in their body’s ability to process blood glucose, to a pre-diabetic state, demanding the body for more insulin production.

Immune System

Sleep facilitates the body’s healing process. That’s why we automatically retire to bed when we are having cold or sore throat. Sleep enables the body to conserve energy and resources to boost immune system and fight diseases. Chemicals of immune system, Cytokines, are also sleep-inducers. On the other hand, deprivation of sleep may affect the way the immune system respond to diseases in the human body.

Cardiovascular Diseases

One of the consequential effects of sleep deprivation is cardiovascular disease. Inadequate sleep over a longer period results in a raised blood pressure and heart rate. This can lead to an increased risk of stroke. People are vulnerable to coronary heart disease when they are not having adequate sleep.


Different researches and studies show that sleep is an essential part of human life. On the contrary, sleep deprivation results can lead to different illnesses such as diabetes and heart problem. On an average, a person should get 7-9 hours of sleep to stay healthy and mentally active.

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