Consciousness 141natural cycles of activity that the body must go through. Some biological rhythms are
monthly, like the cycle of a woman’s menstruation, whereas others are far shorter—the
beat of the heart is a biological rhythm. But many biological rhythms take place on a
daily basis, like the rise and fall of blood pressure and body temperature or the produc-
tion of certain body chemicals (Moore-Ede et al., 1982). The most obvious of these is the
sleep–wake cycle (Baehr et al., 2000).
THE RHYTHMS OF LIFE: CIRCADIAN RHYTHMS The sleep–wake cycle is a circadian
rhythm. The term actually comes from two Latin words, circa (“about”) and diem
(“day”). So a circadian rhythm is a cycle that takes “about a day” to complete.
For most people, this means that they will experience several hours of sleep at least
once during every 24-hour period. The sleep–wake cycle is ultimately controlled by the
brain, specifically by an area within the hypothalamus, the tiny section of the brain that
influences the glandular system. to Learning Objective 2.11.
There was a big fuss over something called melatonin a few years
ago—isn’t melatonin supposed to make people sleep?THE ROLE OF THE HYPOTHALAMUS: THE MIGHTY MITE A lot of people were buy-
ing supplements of melatonin (a hormone normally secreted by the pineal gland)
several years ago, hoping to sleep better and perhaps even slow the effects of aging
( Folkard et al., 1993; Herxheimer & Petrie, 2001; Young, 1996). The release of melatonin
is influenced by a structure deep within the tiny hypothalamus in an area called the
suprachiasmatic (SOO-prah-ki-AS-ma-tik) nucleus, the internal clock that tells people
when to wake up and when to fall asleep (Gandhi et al., 2015; Quintero et al., 2003;
Ya m a g u c h i e t a l. , 2 0 0 3 ; Z i s a p e l , 2 0 0 1 ). T h e s u p r a c h i a s m a t i c n u c l e u s , o r S C N , i s s e n s i-
tive to changes in light. As daylight fades, the SCN tells the pineal gland (located in the
base of the brain) to secrete melatonin (Bondarenko, 2004; Delagrange & Guardiola-
Lemaitre, 1997). As melatonin accumulates, a person will feel sleepy. As the light com-
ing into the eyes increases (as it does in the morning), the SCN tells the pineal gland to
stop secreting melatonin, allowing the body to awaken. That’s a lot of control for such
a small part of the brain.
Melatonin supplements are often used to treat a condition called jet lag, in which
the body’s circadian rhythm has been disrupted by traveling to another time zone. There
is some evidence that melatonin may be linked to a healthier metabolism ( Cardinali
et al., 2013; Gandhi et al., 2015). It may help people who suffer from sleep problems
due to shift work. Shift-work sleep problems, often attributed to the custom of having
workers change shifts against their natural circadian rhythms (e.g., from a day shift to a
night shift, and then back again to an evening shift), have been linked to increased acci-
dent rates, increased absence from work due to illness, and lowered productivity rates
( Folkard & Tucker, 2003; Folkard et al., 1993; Folkard et al., 2005). In addition to mela-
tonin supplements, it has been found that gradually changing the shifts that workers
take according to the natural cycle of the day (e.g., from day shift to evening shift to night
shift, rather than from day shift directly to night shift) has significantly reduced the prob-
lems ( Czeisler et al., 1982; Folkard et al., 2006).
Melatonin is not the whole story, of course. Several neurotransmitters are associ-
ated with arousal and sleep regulation, including serotonin. It was once theorized that
serotonin promoted sleepiness. However, it is not that simple. Serotonin-producing neu-
rons are most active during wakefulness, less active during deep sleep, and relatively
inactive during the type of sleep in which dreams typically occur (Elmenhorst et al.,
2012; Hornung, 2012; Siegel, 2011). Furthermore, effects differ based on which serotonin-
producing cells are firing and which brain structures are receiving those messages. Last,
some serotonin receptors are excitatory and others are inhibitory. For example, some
circadian rhythm
a cycle of bodily rhythm that occurs
over a 24-hour period.Sleep, according to Webb (1992), is the
“gentle tyrant.” As this picture shows, when
the urge to sleep comes upon a person,
it can be very difficult to resist—no matter
where that person is at the time. Can you
think of a time or place when you fell asleep
without meaning to do so? Why do you
think it happened?