144 CHAPTER 4
such as the opossum can afford to sleep during the day and be active at night (when their
food sources are available), because they are protected from predators by sleeping high
up in trees (see Figure 4.2).
THE RESTORATIVE THEORY OF SLEEP The other major theory of why organisms sleep
is called restorative theory, which states that sleep is necessary to the physical health
of the body. During sleep, chemicals that were used up during the day’s activities are
replenished, other chemicals that were secreted in excess and could become toxic if left in
the system are removed, and cellular damage is repaired (Adam, 1980; Moldofsky, 1995;
Xie et al., 2013). As discussed earlier, brain plasticity is enhanced by sleep, and there is
evidence that most bodily growth and repair occur during the deepest stages of sleep,
when enzymes responsible for these functions are secreted in higher amounts (Saper
et al., 2001).
Sleep is also important for forming memories. Studies have shown that the physical
changes in the brain that occur when we form memories are strengthened during sleep,
and particularly so for children (Racsmány et al., 2010; Wilhelm et al., 2013).
to Learning Objective 6.14. This memory effect is no doubt due, at least in part, to the
finding that sleep enhances the synaptic connections among neurons, thus increasing
the plasticity of the brain—the brain’s ability to adapt to experiences (Aton et al., 2009;
Bushey et al., 2011; Cirelli et al., 2012; Frank & Benington, 2006). Sleep may also reduce
the activity of neurons associated with forgetting, leading to memory retention (Berry
et al., 2015), and people who learn tasks right before they go to sleep are able to both
recall and perform those tasks better than if they had not slept after learning (Kurdziel
et al., 2013; Stickgold & Ellenbogen, 2008). to Learning Objective 2.4.
Which of these theories is correct? The answer is that both are probably needed
to understand why sleep occurs the way it does. Adaptive theory explains why people
sleep when they do, and restorative theory (including the important function of memory
formation) explains why people need to sleep.SLEEP DEPRIVATION While we’ve already discussed the importance of being able to
sleep and the dangers of microsleeps, just how much sleep loss can occur before seri-
ous problems start to happen? What will losing out on just one night’s sleep do to arestorative theory
theory of sleep proposing that sleep is
necessary to the physical health of the
body and serves to replenish chemi-
cals and repair cellular damage.
Figure 4.2 Animals and the Adaptive Theory of Sleep