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Module 1.5b

®
AP Science Practice Check Your Understanding

Examine the Concept Apply the Concept
▶ Explain the differences among the stages of sleep. ▶ Would you consider yourself a night owl or a morning lark?
▶ Explain the role of the suprachiasmatic nucleus in sleep. Explain how this relates to your circadian rhythm.
▶ Explain how REM sleep relates to dreaming.

Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
Why Do We Sleep?

1.5-7 What are sleep’s functions?
1.5-7 What ar e sleep’s functions?
Copyright © Bedford, Freeman & Worth Publishers.
As we’ve just seen, our sleep patterns differ from person to person and from culture to cul-
ture. But why do we have this need for sleep? Psychologists offer six possible reasons:
1. Sleep protects. When darkness shut down the day’s hunting, gathering, and travel, our

distant ancestors were better off asleep in a cave, out of harm’s way. Those who didn’t
wander around dark cliffs were more likely to leave descendants. This fits a broad-
er 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 1.5-9.)

Kruglov_Orda/Shutterstock Andrew D. Myers Utekhina Anna/Shutterstock Steffen Foerster/Shutterstock RubberBall Productions/Getty Eric Isselee/Shutterstock pandapaw/Shutterstock

20 hours 16 hours 12 hours 10 hours 8 hours Images 4 hours 2 hours

2. Sleep restores. Sleep gives your body and brain the chance to repair, rewire, and Figure 1.5-9
reorganize. It helps the body heal from infection and restores the immune system Animal sleep time
( Dimitrov et al., 2019 ). Sleep gives resting neurons time to repair themselves, while Would you rather be a brown bat
pruning or weakening unused connections ( Ascády & Harris, 2017 ; Ding et al., 2016; Li that sleeps 20 hours a day or a
et al., 2017 ). Bats and other animals with high waking metabolism burn a lot of calories, giraffe that sleeps 2 hours a day?
producing free radicals, molecules that are toxic to neurons. Sleep sweeps away this toxic (Data from National Institutes of
waste, along with protein fragments that for humans can cause Alzheimer’s disease Health [ NIH], 2010 .)
( Beil, 2018 ; Xie et al., 2013 ). Imagine that when consciousness leaves your house, clean-
ers come in and say, “Good night. Sleep tidy.”
3. Sleep aids memory consolidation. Sleep helps restore and rebuild our fading mem-
ories of the day’s experiences. Our memories are consolidated during slow-wave deep
sleep, by replaying recent learning and strengthening neural connections ( Paller &
Oudiette, 2018 ; Todorva & Zugaro, 2019). Sleep reactivates recent experiences stored
in the hippocampus and moves them to permanent storage elsewhere in the cortex
( Racsmány et al., 2010 ; Urbain et al., 2016). In consequence, adults and children trained
to perform tasks recall them better after a night’s sleep, or even after a short nap, than
after several hours awake ( He et al., 2020 ; Seehagen et al., 2015 ). Older adults’ more
frequently disrupted sleep also disrupts their memory consolidation ( Boyce et al., 2016 ;
Pace-Schott & Spencer, 2011 ).

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