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What Is Sleep? Definitions Of Sleep


Without any prior knowledge about sleep; what an average person would know about sleep is that it is a state in which we are unaware of things around us, that it usually happens at night, that we dream during sleep, that we grow more tired the longer we are awake and that we feel refreshed after a period of sleeping.

To define sleep we need more details and scientifically-proven facts. Here we present different non-medical and medical definitions of sleep.

Dictionary definitions of sleep

Merriam-Webster offers a minimalistic definition:

“the natural periodic suspension of consciousness during which the powers of the body are restored”.

The MacMillan Dictionary also gives a simple, yet more detailed explanation:

“a natural state in which you are unconscious for a time and your body rests,  especially for several hours at night”.

In the Oxford English Dictionary, sleep is defined in scientific detail, as follows:

“a condition of body and mind which typically recurs for several hours every night, in which the nervous system is inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended”.

All of these definitions can successfully convey the meaning of sleep to someone who is wondering about the basic meaning of sleep. The Oxford dictionary has paid the most attention to giving an easy-to-understand, but also substantial, definition.

Medical definitions of sleep

Looking at medical dictionaries, we find somewhat more information explained in medical vocabulary.

Stedman’s Medical Dictionary defines sleep as:

“a natural periodic state of rest for the mind and body, in which the eyes usually close and consciousness is completely or partially lost, so that there is a decrease in bodily movement and responsiveness to external stimuli”.

Taber’s Medical Dictionary offers:

“a periodic state of rest accompanied by varying degrees of unconsciousness and relative inactivity.”

The Free Medical Dictionary offers the longest and fullest of the definitions:

“a period of rest for the body and mind, during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended. Sleep has also been described as a behavioral state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli.”

The Encyclopedia Britannica defines sleep as:

“a normal, reversible, recurrent state of reduced responsiveness to external stimulation that is accompanied by complex and predictable changes in physiology.”

And then goes on to describe the physiological changes, from brain activity, to how sleep affects the entire body.

A unified definition of sleep

If we take these definitions together and extract all sleep-defining information, we get a list of sleep characteristics like this one.

  • natural
  • unconsciousness
  • unresponsiveness to outside stimuli
  • reversible
  • restful
  • restorative
  • eyes are closed
  • the body doesn’t move or is paralyzed
  • usually occurs at night
  • usually lasts for several hours
  • predictable

Putting all these into one, we could make a unified definition:

“sleep is a natural reversible process of rest, typically occurring every night and lasting for several hours, in which an individual is of limited consciousness and unresponsive to outside stimuli, with the eyes closed and a relaxed body; a process during which predictable and complex changes occur.”

About sleep

Sleep is characterized by many things, scientists can observe sleep stages, physiological changes, mental aspects, and others. We can also discuss how much sleep we should get or wonder what the purpose of sleep is.

Sleep stages

Our sleep can be divided into four stages that repeat cyclically throughout the night. Each cycle lasts for about 90 minutes.

The four stages are stage 1 and stage 2; which make up light sleep, slow wave sleep (deep, restorative sleep) and REM (rapid eye movement sleep). Therefore, there are three non-REM stages and one REM stage. Each of these stages is characterized by different brainwaves, body temperature, and level of responsiveness to outside stimuli. Adults spend most of their sleep in stage 2, whereas children have a lot more deep sleep, because, during deep sleep, the human growth hormone is released.

Physiological changes

As our bodies are physically inactive during sleep, our heart rate drops and blood pressure decreases, breathing slows down, core body temperature gets lower, and brain activity changes.

As we are the most detached from reality during deep sleep, our body is the ‘calmest’, breathing the slowest and temperature the lowest. However, in REM sleep, all these physiological aspects change, as we start getting vivid dreams and get emotionally involved, our heart rate increases, eyes move rapidly and all functions increase – except for the body muscles, which are paralyzed to prevent us from enacting our dreams and potentially hurting ourselves.

While we are in non-REM (NREM) sleep, our brain produces a certain type of waves called sleep spindles. They serve many functions, one of which is distorting the outside stimuli such as noise so that we are not woken up easily. When a slight change around or to us happens, spindles occur as a reaction, and less of the stimuli actually reaches our auditory, visual or motor cortex. If a stimulus is too strong, we will wake up. This is how spindles help us have tight sleep. They also help in memory consolidation – remembering and forgetting, and in young brain development.

Other physiological processes involve the replenishment of neurotransmitters, hormone release, and immune system strengthening. Sleep ensures balance (homeostasis) of almost every process in our body.

Mood, learning, memory, and sleep

With good, restorative sleep, we feel rested, energetic, and content. Our mood is much better after a good night’s sleep than following sleep deprivation. This is because of the role chemicals play in our brain (like neurotransmitters – dopamine and serotonin are replenished, making us feel good). These chemicals get completely imbalanced if we don’t have enough sleep.

Our learning and memory are affected by sleep. If we sleep well, the information we’ve gathered during that day, along with the information from before, is consolidated – it goes from short-term memory into long-term memory. Some neural links (synapses) are strengthened, whereas other are diminished. Many studies have shown that sleep helps us learn languages, solve problems, and strengthen motor skills.

A good night’s rest ensures good concentration, mental freshness and an ability to learn easily.

How much should we sleep?

The amount of sleep we need mostly depends on age, but it is also linked to sex (women tend to sleep longer than men), health, and personal needs.

Young children typically need about 9-13 hours of sleep, whereas teenagers can feel fully rested with 8-10 hours. Sleep needs of adults are smaller, so a working person usually finds 7-9 hours enough. The elderly sleep somewhat less on average, with 7-8 hours per night.

All of these hours are just average and the National Sleep Foundation allows one to two hours more or less than posted. This means that 6 hours of sleep might be enough for a small number of adults, but most will be sleep-deprived even with 7 hours.

Why do we sleep?

Scientists haven’t yet found one single reason for why we sleep, but there have been many theories based on what we know about sleep so far.

Some claim that we need sleep in order to conserve energy; others that our body needs restoration and maintenance, so we need to stop being active for that. Another theory says that our brain needs development (this especially goes for children, who spend a lot of time sleeping) as new neural connections are formed.

The real reason might be more complex – it is possible we sleep for a variety of reasons, all of which are equally important.

Additional resources

  1. Foulkes D.  Dang-Vu T.T, et al. Sleep. Biology. Encyclopedia Britannica. Updated January 11, 2019. https://www.britannica.com/science/sleep Accessed on February 3, 2019.
  2. The Characteristics of Sleep. Healthy Sleep. Harvard. December 18, 2007. http://healthysleep.med.harvard.edu/healthy/science/what/characteristics Accessed on February 3, 2019.

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