We first consider evidence that proved the existence of a biological clock: how the clock keeps time and how it regulates our behavior. Because environmental cues are not always consistent, we examine how biological clocks help us interpret environmental cues in an adaptive way.

Biorhythms, the inherent timing mechanisms that control or initiate various biological processes, are linked to the cycles of days and seasons produced by Earth’s rotation on its axis and by its progression in orbit around the sun (Figure 13-1). Earth rotates on its axis once every 24 hours, producing a 24-hour cycle of day and night. The day–night cycle changes across the seasons, however.

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Earth’s axis is tilted slightly, so as it orbits the sun once each year, the North and South Poles incline slightly toward the sun for part of the year and slightly away from it for the rest of the year. As the Southern Hemisphere inclines toward the sun, its inhabitants experience summer: more direct sunshine for more hours each day, and the weather is warmer. At the same time inhabitants of the Northern Hemisphere, inclined away from the sun, experience winter: less direct sunlight, making the days shorter and the weather colder. Over the year the polar inclinations reverse, as do the seasons. Tropical regions near the equator undergo little seasonal or day length change as Earth progresses around the sun.

Daily and seasonal changes have combined effects on organisms, inasmuch as the onset and duration of daily change depend on the season and latitude. Animals living in polar regions have to cope with greater seasonal fluctuations in daily temperature, light, and food availability than do animals living near the equator.

We humans largely evolved as equatorial animals, and our behavior is dominated by a circadian rhythm of daylight activity and nocturnal sleep. Nevertheless our daily cycles adapt to extreme latitudes. Not only does human waking and sleep behavior cycle daily; so also do pulse rate, blood pressure, body temperature, rate of cell division, blood cell count, alertness, urine composition, metabolic rate, sexual drive, feeding behavior, and responsiveness to medications. The activity of nearly every cell in our bodies shares a daily rhythm.

Biorhythms are not unique to animals. Plants display rhythmic behavior exemplified by species whose leaves or flowers open during the day and close at night. Even unicellular algae and fungi display rhythmic behaviors related to the passage of the day. Some animals, including lizards and crabs, change color in a rhythmic pattern. The Florida chameleon, for example, turns green at night, whereas its coloration matches its environment during the day. In short, almost every living organism and every living cell displays rhythms related to daily changes (Bosler et al., 2015).

If animal behavior were affected only by daily changes in external cues, the neural mechanisms that account for changes in behavior would be simple to study. An external cue—say, sunrise—could be isolated and the neural processes that respond to the cue identified.

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That behavior is not driven simply by external cues was first recognized in 1729 by the French geologist Jean Jacques d’Ortous de Mairan (see Raven et al., 1992). In an experiment similar to the one illustrated in the Procedure section of Experiment 13-1, de Mairan isolated a plant from daily light, dark, and temperature cues. He noted that the rhythmic movements of its leaves seen over a light–dark cycle continued when it was isolated as graphed in the Results section of the experiment.

What concerned investigators who came after de Mairan was the possibility that some undetected external cue stimulates the plant’s rhythmic behavior. Such cues could include changes in temperature, in electromagnetic fields, and even in the intensity of cosmic rays from outer space. But further experiments showed that daily fluctuations are endogenous—they come from within the plant. Thus, the plant must have a biological clock. The contributions of endogenous rhythms to plants is important both to agriculture and to food storage (Bendix et al., 2015).

Similar experiments show that almost all organisms, including humans, have biological clocks that synchronize behavior to the temporal passage of a real day and make predictions about tomorrow. A biological clock signals that if daylight lasts for a given time today, it will last for about the same time tomorrow. A biological clock allows us to anticipate events and prepare for them both physiologically and cognitively. And unless external factors get in the way, a biological clock regulates feeding times, sleeping times, and metabolic activity as appropriate to day–night cycles. Biological clocks also produce epigenetic effects: they regulate gene expression in every cell in the body (Zhang et al., 2014).

Although the existence of endogenous biological clocks was demonstrated nearly 300 years ago, detailed study of biorhythms had to await the development of electrical and computer-based timing devices. Behavioral analysis requires a method for counting behavioral events and a method for displaying those events in a meaningful way. For example, rodent behavior was first measured by giving the animal access to a running wheel for exercise (Figure 13-2A).

A computer records each turn of the wheel and displays the result (Figure 13-2B). Because most rodents are nocturnal, sleeping during light hours and becoming active during dark hours, their wheel running takes place in the dark. If each day’s activity is plotted under the preceding day’s activity in a column, we observe a pattern—a cycle of activity over time. A glance at the pattern reveals when and how active the animal is (Figure 13-2C).

The animal’s activity cycle has a period, the time required to complete one cycle of activity. Most animals’ activity period is about 24 hours in an environment in which the lights go on and off regularly. Our own sleep–wake period also is about 24 hours. The measurement of periods and the events that control them are central to understanding circadian rhythms.

Many behaviors have periods longer or shorter than this 24-hour circadian rhythm. Circannual rhythms last about a year. Many animals’ migratory and mating cycles are circannual. Other biorhythms have monthly or seasonal periods greater than a day but less than a year. These are infradian rhythms. The menstrual cycle and associated hormonal changes of human females, with an average period of about 28 days, is an infradian biorhythm linked to the cycle of the moon and thus also referred to as a circalunar cycle (Amariei et al., 2014).

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Ultradian rhythms have a period of less than one day. Our eating behavior, which takes place about every 90 minutes to 2 hours, including snacks, is one ultradian rhythm. Rodents, although active throughout the night, display an ultradian rhythm in being most active at the beginning and end of the dark period.

The fact that a behavior appears to be rhythmic does not mean that it is ruled only by a biological clock. Animals may postpone migrations as long as food supplies last. They adjust their circadian activities in response to the availability of food, the presence of predators, and competition from other members of their own species. We humans obviously change our daily activities in response to seasonal changes, work schedules, and play opportunities. Therefore, whether a rhythmic behavior is produced by a biological clock and the extent to which it is controlled by a clock must be demonstrated experimentally.

To determine whether a rhythm is produced by a biological clock, researchers design three types of tests in which they manipulate relevant cues, especially light cues. A test is given (1) in continuous light, (2) in continuous darkness, or (3) by choice of the participant. Each treatment yields a slightly different insight into the periods of biological clocks.

Jurgen Aschoff and Rutger Weber first demonstrated that the human sleep–waking rhythm is governed by a biological clock. They allowed participants to select their light–dark cycle and studied them in an underground bunker, where no cues signaled when day began or ended. The participants selected the periods when they were active and when they slept, and they turned the lights on and off at will. In short they selected the length of their own day and night.

Measures of ongoing behavior and recording of sleep periods with sensors on the beds revealed that the participants continued to show daily sleep–activity rhythms. This finding demonstrates that humans have an endogenous biological clock that governs sleep–waking behavior. Figure 13-3 shows, however, that the biorhythm is different when compared with biorhythms before and after isolation. Although the period of the participants’ sleep–wake cycles approximated 24 hours before and after the test, during the test they lengthened to about 25 to 27 hours, depending on the person.

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The participants chose to go to bed 1 to 2 hours later every “night.” Soon they were getting up at about the time the experimenters outside the bunker were going to bed. Clearly. the participants were displaying their own personal cycles. Such a free-running rhythm runs at a frequency of the body’s own devising when environmental cues are absent. Humans’ self-selected free-running rhythm is slightly longer than 24 hours.

The period of free-running rhythms also depends on the light-related biology of the species. When hamsters, a nocturnal species, are tested in constant darkness, their free-running periods are a little shorter than 24 hours; when they are tested in constant light, their free-running periods are a little longer than 24 hours. This test dependency is typical of nocturnal animals. As Figure 13-4 shows, the opposite free-running periods are typical of diurnal animals (Binkley, 1990). When sparrows, diurnal birds, are tested in constant darkness, their free-running periods are a little longer than 24 hours; when they are tested in constant light, their free-running periods are a little shorter than 24 hours.

A rule of thumb to explain the period of free-running rhythms in light or dark is that animals expand and contract their sleep periods as the sleep-related period—light for hamsters and dark for sparrows—expands or contracts. Understanding this point enables us to predict how excess artificial lighting, which expands the light portion of our days, influences our circadian periods. Because we are diurnal, our sleep periods contract and we get less sleep each night.

Endogenous rhythmicity is not the only factor that contributes to circadian periods. A mechanism exists for setting rhythms to correspond to environmental events as well. To be useful, the biological clock must keep to a time that predicts actual changes in the day–night cycle. If a biological clock is like a slightly defective wristwatch, it will eventually provide times that are inaccurate by hours and so be useless.

If we reset an errant wristwatch each day, however—say, when we awaken—it provides useful information even though it is not perfectly accurate. Equivalent ways of resetting a free-running biological clock include sunrise and sunset, eating times, and many other activities that influence the period of the circadian clock.

Aschoff and Weber called a clock-setting cue a Zeitgeber (time giver in German). When a Zeitgeber resets a biorhythm, the rhythm is said to be entrained. Light is the most potent entraining stimulus. Clinical Focus 13-2, Seasonal Affective Disorder, explains its importance in entraining circadian rhythms.

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The property that allows a biological clock to be entrained explains how circadian rhythms synchronize with seasonal changes in day–night duration. North and south of the equator the time of onset and the length of day and night change as the seasons progress. At extreme latitudes daylight begins very early in the morning in summer and very late in the morning in winter. An entrained biological clock allows an animal to synchronize its daily activity across these seasonal changes.

A biological clock that resets each day tells an animal that daylight will begin tomorrow at approximately the same time that it began today and that tomorrow will last approximately as long as today did. Current research finds that light Zeitgebers are effective at both sunrise and sunset: morning light sets the biological clock by advancing it, and evening darkness sets the clock by retarding it (Schmal et al., 2015).

The potent entraining effect of light Zeitgebers is illustrated by laboratory studies of Syrian hamsters, perhaps one of the most compulsive animal timekeepers. When given access to running wheels, hamsters exercise during the night segment of the laboratory day–night cycle. A single brief flash of light is an effective Zeitgeber for entraining their biological clocks.

Considering the less compulsive behavior that most of us display, we should shudder at the way we entrain our own clocks when we stay up late in artificial light, sleep late some days, and get up early by using an alarm clock on other days. Light pollution, the extent to which we are exposed to artificial lighting, disrupts circadian rhythms and accounts for a great deal of inconsistent behavior associated with accidents, daytime fatigue, alterations in emotional states, obesity, diabetes, and other disorders characteristic of metabolic syndrome described in Clinical Focus 13-1, Doing the Right Thing at the Right Time (Gerhart-Hines & Lazar, 2015).

Entrainment works best if the adjustment made to the biological clock is not too large. People who work night shifts are often subject to huge adjustments, especially when they work the graveyard shift (11:00 P.M. to 7:00 A.M.), the period when they would normally sleep. Study results show that adapting to such a change is difficult and stressful, and it increases susceptibility to disease by altering immune system rhythms (Labrecque & Cermakian, 2015). Compared with people who have a regular daytime work schedule, people who work night shifts have a higher incidence of metabolic syndrome. Thus, shift workers benefit from vigilance in maintaining good sleep habits and diet and in exercising to minimize other risk factors for metabolic syndrome. Adaptations to shift work fare better if people first work the swing shift (3:00 P.M. to 11:00 P.M.) for a time before beginning the graveyard shift.

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Long-distance air travel—say from North America to Europe or Asia—also demands large and difficult time adjustments. For example, travelers flying east from New York to Paris begin their first day in Europe just when their biological clock is signaling that it is time for sleep (Figure 13-5). The difference between a person’s circadian rhythm and the daylight cycle in a new environment can produce the disorientation and fatigue of jet lag.

The west-to-east traveler generally has a more difficult adjustment than does the east-to-west traveler, who needs to stay up only a little longer than usual. The occasional traveler may cope with jet lag quite well, but frequent travelers such as airline personnel face a substantial adaptive challenge. The occasional traveler can manage jet lag with sleep on arrival or shortly after. The brain’s biological clock resets in a day and other body organs follow after about a week. For frequent travelers and flight crews, resetting is not so easy. Persistent asynchronous rhythms generated by jet lag are associated with altered sleep and temperature rhythms, fatigue, and stress, even reduced success by sports teams traveling more than 3 hours from west to east (Weingarten and Collop, 2013).

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A Clock for All Seasons

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.