Why Do People Sleep? Surprisingly, Nobody Knows

By: Jordan Gaines, 2nd year PhD candidate in the Neuroscience Graduate Program

SleepWhy do people eat?

The answer is obvious: to convert food into energy for us to do work. We wouldn’t be able to move or think otherwise, and lack of food would eventually starve us to death.

Now consider: why do people sleep?

According to William C. Dement, renowned sleep researcher and founder of the U.S.’s first sleep laboratory, “the only reason we need to sleep that is really, really solid is because we get sleepy.”

In other words, after decades of research—nobody quite knows.

All mammals, birds, and reptiles have demonstrated signs of regular sleep patterns, notes Allan Rechtschaffen, professor emeritus at the University of Chicago and co-author of The AASM Manual for the Scoring of Sleep and Associated Events used widely by laboratories to report human sleep.

flamingo-74154_640Whether it’s regular, unconscious-like sleep in humans; a dolphin simultaneously swimming and sleeping with half of its brain at a time; or a flamingo balancing on a single leg while taking a snooze—sleep is, for whatever reason, an evolutionarily-conserved event to which we devote roughly 1/3 of our lives.

Characterized by dreaming and quick, random movements of the eyes, we experience four or five stages of REM (rapid eye movement) during a night’s sleep. Since its discovery in 1953, researchers have realized that sleep is organized and active, not a simple off-switch for the brain.

If animals didn’t need sleep, there would be no negative consequences—and, clearly, that’s not the case.

Plus, we know that lack of sleep one night results in compensation the next night. A lower percentage of “restful” slow-wave sleep one night is often followed by an unusually high proportion the following night.

There are three main theories as to why us humans require sleep:

1. Restoration. Sleep deprivation has been shown to alter our immune and endocrine systems, causing delays in wound healing, enhancing our chances of becoming sick, and changing hormone levels, such as those controlling growth and hunger (Jenni et al., 2007).

2. Development. Interestingly, young people experience longer periods of REM than adults. Babies spend up to 80% of their time sleeping in REM (Hobson, 2009), suggesting that deep sleep plays a critical role in physical growth and development.

3. Memory and attention. You know how it’s difficult to remember things and pay attention the day after a night of short sleep? Many studies have confirmed this anecdote, demonstrating decreased reaction time and memory in sleep-deprived humans (Marshall et al., 2006).

Sleepy_Kit_(6969930840)Clearly, sleep is important for many different aspects of our survival and well-being, but questions remain. How much sleep is optimal? Why can I get by on 7 or 8 hours when my dog is perfectly content and healthy sleeping 20 hours a day? How, exactly, is sleep restorative, allowing us to grow and develop?

It seems that Dement’s sentiment is the only true answer, even a decade into the 21st century. Because we get sleepy.

In my next entry, I’ll describe some previous work by my laboratory that delves into the notion of “sleepiness.”


Hobson, J.A. (2009). “REM sleep and dreaming: towards a theory of protoconsciousness”. Nature Reviews 10 (11): 803–813.

Jenni, O.G.; Molinari, L.; Caflisch, J.A.; Largo, R.H. (2007). “Sleep duration from ages 1 to 10 years: Variability and stability in comparison with growth”. Pediatrics120 (4): e769–e776.

Marshall et al., 2006, as cited in Walker, M.P. (2009). “The Role of Sleep in Cognition and Emotion”. Annals of the New York Academy of Sciences 1156: 174.

4 thoughts on “Why Do People Sleep? Surprisingly, Nobody Knows

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