8.2 Why Do We Sleep?
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So, our sleep patterns differ from person to person. But why do we have this need for sleep? Psychologists offer five possible reasons.
Sleep protects. When darkness shut down the day’s hunting, food gathering, and travel, our distant ancestors were better off asleep in a cave, out of harm’s way. Those who didn’t try to navigate around dark cliffs were more likely to leave descendants. This fits a broader principle: A species’ sleep pattern tends to suit its ecological niche (Siegel, 2009). Animals with the greatest need to graze and the least ability to hide tend to sleep less. Animals also sleep less, with no ill effects, during times of mating and migration (Siegel, 2012). (For a sampling of animal sleep times, see FIGURE 8.6.)
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"Sleep faster, we need the pillows."
Yiddish proverb
"Corduroy pillows make headlines."
Anonymous
Sleep helps us recuperate. Sleep helps restore the immune system and repair brain tissue. Bats and other animals with high waking metabolism burn a lot of calories, producing a lot of free radicals, molecules that are toxic to neurons. Sleeping a lot gives resting neurons time to repair themselves, while pruning or weakening unused connections (Gilestro et al., 2009; Tononi & Cirelli, 2013). Sleep also enables house cleaning. Studies of mice show that sleep sweeps the brain of toxic metabolic waste products (Xie et al., 2013). Think of it this way: When consciousness leaves your house, workers come in for a makeover, saying “Good night. Sleep tidy.”
Sleep helps restore and rebuild our fading memories of the day’s experiences. Sleep consolidates our memories. It replays or “reactivates” recent learning and strengthens its neural connections (Yang et al., 2014). Sleep also shifts experiences stored in the hippocampus to permanent storage elsewhere in the cortex (Diekelmann & Born, 2010; Racsmány et al., 2010). Adults and children trained to perform tasks therefore recall them better after a night’s sleep, or even after a short nap, than after several hours awake (Friedrich et al., 2015; Kurdziel et al., 2013; Stickgold & Ellenbogen, 2008). After sleeping well, older people remember more of recently learned material (Drummond, 2010). Sleep, it seems, strengthens memories in a way that being awake does not.
Sleep feeds creative thinking. Dreams can inspire noteworthy artistic and scientific achievements, such as the dreams that clued chemist August Kekulé to the structure of benzene (Ross, 2006) and inspired medical researcher Carl Alving (2011) to invent the vaccine patch. More commonplace is the boost that a complete night’s sleep gives to our thinking and learning. After working on a task, then sleeping on it, people solve difficult problems more insightfully than do those who stay awake (Barrett, 2011; Sio et al., 2013). They also are better at spotting connections among novel pieces of information (Ellenbogen et al., 2007). To think smart and see connections, it often pays to ponder a problem just before bed and then sleep on it.
Sleep supports growth. During deep sleep, the pituitary gland releases a growth hormone that is necessary for muscle development. A regular full night’s sleep can also “dramatically improve your athletic ability,” report James Maas and Rebecca Robbins (2010). Well-
rested athletes have faster reaction times, more energy, and greater endurance. Teams that build 8 to 10 hours of daily sleep into their training show improved performance.
Given all the benefits of sleep, it’s no wonder that sleep loss hits us so hard.
