9.2 Why Do We Sleep?

9-5 What are sleep’s functions?

So, our sleep patterns differ from person to person. But why do we have this need for sleep?

Psychologists believe sleep may exist for five reasons.

  1. 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 9.6.)
    Figure 9.6
    Animal sleep time Would you rather be a brown bat and sleep 20 hours a day or a giraffe and sleep 2 hours a day? (Data from NIH, 2010.)
  2. Sleep helps us recuperate. It 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.”
  3. Sleep helps restore and rebuild our fading memories of the day’s experiences. Sleep consolidates our memories. It reactivates recent experiences stored in the hippocampus and shifts them for 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 (Kurdziel et al., 2013; Stickgold & Ellenbogen, 2008). Among older adults, more frequently disrupted sleep also disrupts memory consolidation (Pace-Shott & Spencer, 2011). 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.
  4. 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 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.
  5. Sleep supports growth. During deep sleep, the pituitary gland releases a growth hormone that is necessary for muscle development. As we age, we release less of this hormone and spend less time in deep sleep (Pekkanen, 1982).

“Sleep faster, we need the pillows.”

Yiddish proverb

“Corduroy pillows make headlines.”

Anonymous

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Ample sleep supports skill learning and high performance This was the eperience of Olympic gold medalist Srah Hughes.

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, and teams that build 8 to 10 hours of daily sleep into their training show improved performance. One study observed Stanford University men’s basketball players’ performance for about three weeks. During an ensuing five to seven weeks of extended sleep—aiming for 10 hours in bed—their average sleep increased 110 minutes per night, their sprint times decreased, and their free throw and 3-point shooting percentages both increased 9 percent (Mah et al., 2011). Top violinists also report sleeping 8.5 hours a day on average, and rate practice and sleep as the two most important improvement-fostering activities (Ericsson et al., 1993).

Slow-wave sleep, which occurs mostly in the first half of a night’s sleep, produces the human growth hormone necessary for muscle development. REM sleep and NREM-2 sleep, which occur mostly in the final hours of a long night’s sleep, help strengthen the neural connections that build enduring memories, including the “muscle memories” learned while practicing tennis or shooting baskets.

The optimal exercise time is late afternoon or early evening, Maas and Robbins advise, when the body’s natural cooling is most efficient. Early morning workouts are ill-advised, because they increase the risk of injury and rob athletes of valuable sleep. Heavy workouts within three hours of bedtime should also be avoided because the arousal disrupts falling asleep. Precision muscle training, such as shooting free throws or piano playing, benefits when practiced shortly before sleep (Holz et al., 2012; Tamaki et al., 2013).

Maas has been a sleep consultant for college and professional athletes and teams. On his advice, basketball’s Orlando Magic cut early morning practices. He also advised one young woman, Sarah Hughes, who felt stymied in her efforts to excel in figure-skating competition. “Cut the early morning practice,” he instructed, as part of the recommended sleep regimen. Soon thereafter, Hughes’ performance scores increased, ultimately culminating in her 2002 Olympic gold medal. Given all the benefits of sleep, it’s no wonder that sleep loss hits us so hard.

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RETRIEVAL PRACTICE

  • What are five proposed reasons for our need for sleep?

(1) Sleep has survival value. (2) Sleep helps us restore and repair brain tissue. (3) During sleep we consolidate memories. (4) Sleep fuels creativity. (5) Sleep plays a role in the growth process.