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Melatonin production suppressed Melatonin produced
Suprachiasmatic Suprachiasmatic
nucleus nucleus

Pineal gland Pineal gland
Light
No melatonin Melatonin
produced produced

Blood vessel Blood vessel

(a) (b)
Figure 1.5-8
The biological clock
(a) Light striking the retina signals the suprachiasmatic nucleus (SCN) to suppress the pineal gland’s production of the sleep hormone melatonin.
(b) At night, the SCN quiets down, allowing the pineal gland to release melatonin into the bloodstream.

With sleep, as with waking behavior, biology and environment interact. Thanks to mod-
CONNECT 1.5-6 ern lighting, shift work, and social media diversions, many people who might have gone
to bed at 9:00 p.m. in days past are now up until 11:00 p.m. or later. Whether for work or
Copyright © Bedford, Freeman & Worth Publishers.
Students should recall from Module play, bright light affects our sleepiness by activating light-sensitive retinal proteins. This
1.4b that the hypothalamus is signals the brain’s suprachiasmatic nucleus (SCN) to decrease production of melatonin,
responsible for behaviors and mental a sleep-inducing hormone found in the hypothalamus (Chang et al., 2015; Gandhi et al.,
processes governed by hormones, 2015) (Figure 1.5-8). (A 2017 Nobel Prize was awarded for research on the molecular biology
that runs our biological clock.)
such as eating, sex, and the stress Being bathed in (or deprived of) light disrupts our 24-hour biological clock (Czeisler

response. This module reinforces et al., 1999; Dement, 1999). Imposed stay-at-home orders during the Covid pandemic
led people in many countries to experience lower-than-normal levels of light (María
that idea by showing that yet another et al., 2020). Night-shift workers may experience a chronic state of desynchronization. As
behavior governed by hormones— a result, they become more likely to develop fatigue, stomach problems, heart disease,
sleep—is regulated by the suprachias- and, for women, breast cancer (Knutsson & Bøggild, 2010; Lin et al., 2015; Puttonen
et al., 2009).
matic nucleus, a small section of the Our ancestors’ body clocks were attuned to the rising and setting Sun of the 24-hour
hypothalamus. day, leading them to get more sleep during the dark winter months and less during the
sunny summer months (van Egmond et al., 2019). Today’s young adults adopt something
closer to a 25-hour day, by staying up too late to get 8 hours of sleep. Approximately
TEACH 1.5-6 Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
90 percent of Americans report using a light-emitting electronic device one hour before
Enrichment going to sleep (Chang et al., 2015). Such artificial light delays sleep and affects sleep
quality. This phenomenon was seen in first-year college students who stayed up late for
Tell your students that, in contrast entertainment viewing, which interfered with the onset, quality, and duration of their
to the sleep induced by most drugs, suprachiasmatic nucleus sleep (Exelmans & Van den Bulck, 2018). Streaming disrupts dreaming.
(SCN) a pair of cell clusters in
Sleep often eludes those who stay up late and sleep in on weekends, then go to bed ear-
sleep induced by melatonin supple- the hypothalamus that controls lier on Sunday to prepare for the new school week (Oren & Terman, 1998). Like New Yorkers
circadian rhythm. In response to
ments seems remarkably normal. The light, the SCN adjusts melatonin readjusting after a trip to California, they experience a kind of jet lag. For North Americans who
stages of both non-REM and REM production, thus modifying our fly to Europe and need to be up when their circadian rhythm cries “SLEEP,” bright light (spend-
ing the next day outdoors) helps reset the biological clock (Czeisler et al., 1986, 1989; Eastman
feelings of sleepiness.
occur at the usual hours and last the et al., 1995).
usual amount of time, and most users
do not seem to feel drowsy or expe- 98 Unit 1 Biological Bases of Behavior
rience side effects. The data suggest
that melatonin may be helpful for
people who: TEACH 1.5-6 15/12/23 9:24 AM
03_myersAPpsychology4e_28116_ch01_002_163.indd 98
• need to sleep during the day Active Learning
(night-shift workers or those who
fly across time zones). (10 minutes) Have students engage in a
directed paraphrasing exercise by explaining
• are taking medications that inhibit the influence of the hypothalamus and mel-
normal melatonin synthesis. atonin on sleep to a first grader. Have them
• are patients with diseases that share their explanations with the class, and
cause insomnia. clear up any confusion before moving on.
Information from Haimov, I., & Lavie, P. (1996).
Melatonin: A soporific hormone. Current Directions
in Psychological Science, 5, 106–111.

98 Unit 1 Biological Bases of Behavior

03_HammerTE4e_47547_ch01_2a_163_4pp.indd 98 07/02/24 5:25 PM