142 CHAPTER 4
receptors facilitate some stages of sleep, while others inhibit other stages (Siegel, 2011;
Zhang et al., 2015).
Body temperature plays a part in inducing sleep, too. The suprachiasmatic nucleus,
as part of the hypothalamus, controls body temperature. The higher the body tempera-
ture, the more alert people are; the lower the temperature, the sleepier they are. When
people are asleep at night, their body temperature is at its lowest level. Be careful: The
research on the effects of serotonin and body temperature on sleep is correlational, so we
cannot assume causation, and there are many different factors involved in sleep.
to Learning Objective 1.7.
In studies in which volunteers spend several days without access to information
about day or night, their sleep–wake cycles lengthened (Czeisler, 1995; Czeisler et al.,
1980). The daily activities of their bodies—such as sleeping, waking, waste production,
blood pressure rise and fall, and so on—took place over a period of 25 hours rather than
24 hours. Our circadian rhythms are synchronized to a 24-hour day consistent with the
day-night cycle due to the suprachiasmatic nucleus, which receives direct input from
some retinal ganglion cells responding to light (McCormick & Westbrook, 2013).
In the same studies, body temperature dropped consistently even in the absence
of light (Czeisler et al., 1980). As body temperature dropped, sleep began, giving further
support to the importance of body temperature in the regulation of sleep.
Why We Sleep
4.4 Explain why we sleep.
How much sleep is enough sleep? The answer varies from person to person because of
each person’s age and possibly inherited sleep needs (Feroah et al., 2004), but most young
adults need about 7–9 hours of sleep each 24-hour period in order to function well (see
Figure 4.1). Some people are short sleepers, needing only 4 or 5 hours, whereas others are
long sleepers and require more than 9 hours of sleep (McCann & Stewin, 1988). As we
age, we seem to sleep less during each night until the average length of sleep approaches
only 6 hours. As sleep researcher Dr. Jerry Siegel describes in the video How Much Sleep
Do We Need?, the amount of sleep that we get can have an impact on our health.
CC
Watch the V i d e o How Much Sleep Do We Need?
