An average human sleeps for a third of his or her life (1). Other animals can display great variation in their time spent sleeping, ranging from the koala (up to 22 hours) to the giraffe (as little as 1.5 hours) (1).
Numerous studies affirm the importance of sleep to health and well-being, in a manner comparable to that of food (2). A human will, on average, die more quickly from sleep deprivation than from food deprivation (1). After all, an average person can last about two weeks without food (if also supplied with water); he or she would last only 10 days without sleep (1).
To evaluate the importance of sleep, one may see what happens when it is deprived. In the short term, performance and alertness drop—a reduction of 1.5 hours of sleep can reduce alertness by as much as 32% (3). Memory formation and cognition are impaired as well (3).
In the long term, numerous negative health effects occur; high blood pressure, heart attacks, heart failure, strokes, Attention Deficit Disorder (ADD), depression and other mood disorders, mental impairment, and generally poorer quality of life have all been correlated to long term sleep deprivation (3). In addition, those who sleep less than six hours per night have increased mortality risk—a risk even greater than that of smoking, high blood pressure, and heart disease (3).
The effects of total sleep deprivation (TSD) have been examined in animals. In one study at the University of Chicago, a group of rats were subjected to TSD. All rats died or were sacrificed when death appeared imminent. The rats displayed no anatomical causes of death, though they did possess very debilitated appearances, lesions on tails and paws, as well as weight loss despite increased food intake (4).
Sleepiness is a natural protective mechanism comparable to hunger, though hunger fulfills a physical necessity for nutrients and proper metabolism (2). Both are regulated by powerful internal drives that discomfort the organism even before sleep or food deprivation has a marked physiological effect.
Though scientists understand sleep intimately from years of study, the question of why sleep exists still has not been definitively answered (2). They have, however, developed many theories over the years. The complete answer likely lies in some combination of these theories.
First, Inactivity Theory (also called the adaptive/evolutionary theory) explains that inactivity at night or day is an adaptation for survival at a time of daily vulnerability through low visibility or oppressive temperature (2). For example, animals may sleep through periods of time during which they are most often preyed upon.
Through natural selection, those animals that stayed quiet through these times survived in greater proportions and eventually came to the mechanism of sleep (2). However, some have challenged this theory, asserting that it would be safer to remain conscious to react to emergencies (2).
Second, the Energy Conservation Theory proposes reduction of energy demand and expenditure is aided by a period of inactivity such as sleep. In natural selection, competition for and efficient use of energy resources is vital—if a species uses its given energy resources even slightly more effectively, it would have an advantage over similar species (2).
Especially if this period occurs during a part of the day or night with little opportunity to obtain food, energy metabolism can be reduced. Research shows that energy metabolism is reduced by as much as 10% in humans (and even more in certain other species) (2).
Third, restorative theories propose that sleep restores, repairs, and rejuvenates the body. After all, many major restorative functions—such as muscle growth, tissue repair, protein synthesis, and growth hormone release occur mostly or entirely during sleep (2).
Another segment of the theories explains that neurons need sleep to clear away adenosine accumulated as a byproduct of neuron activity (2) . The build-up of adenosine results in the perception of tiredness and leads to a “drive to sleep” (2). Remaining awake creates increasing amounts of buildup that will eventually become crippling (2).
Even in adults, sleep also assists in memory formation. A study that administered aptitude tests to groups of students demonstrated that those with sleep had better recall of test patterns, even if the sleep came from daytime naps (5).
Finally, the brain plasticity theory suggests that sleep is correlated to changes in structure and organization of the brain. In particular, sleep has a critical role in brain development of infants and young children (2). Sleep also improves learning ability and task performance in adults (2).
Indeed, rats trained to navigate a maze showed the same activity patterns during sleep as when they physically were performing the task. Sleep helped the brain reconstruct the experience and improved the rats’ performance when they ran the mazes again (Stafford).
1. The Better Sleep Council, Sleep Statistics and Research. Available at http://bettersleep.org/better-sleep/the-science-of-sleep/sleep-statistics-research (5 April 2013).
2. Division of Sleep Medicine at Harvard Medical School, Why Do We Sleep, Anyways? (18 December 2007). Available at http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep (1 April 2013).
3. M. Breus, Sleep Habits: More Important Than You Think. Available at http://www.webmd.com/sleep-disorders/features/important-sleep-habits (5 April 2013).
4. C.A. Everson et al., Sleep Deprivation in the rat: III. Total Sleep Deprivation (1989). Available at http://www.ncbi.nlm.nih.gov/pubmed/2928622 (6 April 2013).
5. T. Stafford, Why do we need to sleep? (28 February 2012). Available at http://www.bbc.com/future/story/20120228-why-do-we-need-to-sleep (1 April 2013).