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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
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(a) (b)
Copyright © Bedford, Freeman & Worth Publishers.
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-
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
play, bright light affects our sleepiness by activating light-sensitive retinal proteins. This
signals the brain’s suprachiasmatic nucleus (SCN) to decrease production of melatonin,
a sleep-inducing hormone found in the hypothalamus (Chang et al., 2015; Gandhi et al.,
2015) (Figure 1.5-8). (A 2017 Nobel Prize was awarded for research on the molecular biology
that runs our biological clock.)

Being bathed in (or deprived of) light disrupts our 24-hour biological clock (Czeisler
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
et al., 2020). Night-shift workers may experience a chronic state of desynchronization. As
a result, they become more likely to develop fatigue, stomach problems, heart disease,
and, for women, breast cancer (Knutsson & Bøggild, 2010; Lin et al., 2015; Puttonen
et al., 2009).
Our ancestors’ body clocks were attuned to the rising and setting Sun of the 24-hour
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
90 percent of Americans report using a light-emitting electronic device one hour before
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
entertainment viewing, which interfered with the onset, quality, and duration of their
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-
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 readjusting after a trip to California, they experience a kind of jet lag. For North Americans who
light, the SCN adjusts melatonin
production, thus modifying our fly to Europe and need to be up when their circadian rhythm cries “SLEEP,” bright light (spend-
feelings of sleepiness. ing the next day outdoors) helps reset the biological clock (Czeisler et al., 1986, 1989; Eastman
et al., 1995).

98 Unit 1 Biological Bases of Behavior

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