The Body's Clock Circadian rhythms are controlled by a biological clock, or overall coordinator, located in a tiny cluster of cells in the hypothalamus called the suprachiasmatic nucleus (SCN). Neural pathways from special receptors in the back of the eye transmit information to the SCN and allow it to respond to changes in light and dark. The SCN then sends out messages that cause the brain and body to adapt to these changes. Other clocks also exist, scattered around the body, but for most circadian rhythms, the SCN is regarded as the master pacemaker.

The SCN regulates fluctuating levels of hormones and neurotransmitters, and they in turn provide feedback that affects the SCN's functioning. During the dark hours, one hormone regulated by the SCN, melatonin, is secreted by the pineal gland, deep within the brain. Melatonin induces sleep. When you go to bed in a darkened room, your melatonin level rises; when light fills your room in the morning, it falls. Melatonin, in turn, appears to help keep the biological clock in phase with the light–dark cycle (Haimov & Lavie, 1996; Houdek et al., 2015). What's more, melatonin treatments have been used to regulate the disturbed sleep–wake cycles of blind people who lack light perception and whose melatonin production does not cycle normally (Flynn-Evans et al., 2014). The video Rhythms of Consciousness provides more information on how the brain regulates sleep-wake cycles.

When the Clock Is Out of Sync Under normal conditions, the rhythms governed by the SCN are in phase with one another. Their peaks may occur at different times, but if you know when one rhythm peaks, you can predict fairly well when another will. It's a little like knowing the time in London if you know the time in New York. But when your normal routine changes, your circadian rhythms may be thrown out of phase. Such internal desynchronization often occurs when people take airplane flights across several time zones. Sleep and wake patterns usually adjust quickly, but temperature and hormone cycles can take several days to return to normal. The resulting jet lag affects energy level, mental skills, and motor coordination (Sack, 2010).

Internal desynchronization also occurs when workers must adjust to a new shift. Efficiency drops, the person feels tired and irritable, accidents become more likely, and sleep disturbances and digestive disorders may occur. For police officers, emergency-room personnel, airline pilots, truck drivers, and operators of nuclear power plants, the consequences can be a matter of life and death. Night work itself is not necessarily a problem: With a schedule that always stays the same, even on weekends, people often adapt. However, many swing and night-shift assignments are made on a rotating basis, so a worker's circadian rhythms never have a chance to resynchronize.

Some scientists hope eventually to help rotating-shift workers and travelers crossing time zones adjust more quickly by using melatonin, drugs, or other techniques to “reset the clock” (van de Werken et al., 2013), but so far these techniques do not seem ready for prime time. Giving shift workers melatonin sometimes helps and sometimes does not; stimulant drugs can improve attention but don't eliminate physical fatigue; and although short naps on the job increase alertness, many employers do not like the idea of paying people to sleep (Kolla & Auger, 2011). When a pilot in Nevada was unable to contact a napping air traffic controller at 2:00 a.m., scientists butted heads with government officials about who was to blame. “There should be sanctioned on-shift napping. That's the way to handle night-shift work,” said one neuroscientist. But the Federal Aviation Administration still bans the practice, while calling for more data.

One reason a simple cure for desynchronization has so far eluded scientists may be that circadian rhythms can be affected by illness, stress, exercise, drugs, mealtimes, and many other factors. Also, circadian rhythms differ greatly from person to person. There truly are morning people (“larks”) and evening people (“owls”). Scientists call your identity as a lark or owl your “chronotype.” Genetic influences may contribute to chronotypes, although early attempts to find “chronotype genes” have proven difficult to replicate (Chang et al., 2011; Osland et al., 2011). Moreover, your chronotype may change as you age: Adolescents are more likely than children and older adults to be owlish (Biss & Hasher, 2012), which may be why many teenagers have trouble adjusting to school schedules. You may be able to learn about your own personal pulses through careful self-observation, and you may want to try putting that information to use when planning your daily schedule.

Does the Season Affect Moods? Clinicians report that some people become depressed during particular seasons, typically winter, when periods of daylight are short, a phenomenon that has come to be known as seasonal affective disorder (SAD). This condition is relatively uncommon and is not recognized as an official disorder in the leading diagnostic manual used by clinicians (American Psychiatric Association, 2013). During the winter months, SAD patients report feelings of sadness, lethargy, drowsiness, and a craving for carbohydrates. To counteract the effects of sunless days, physicians and therapists often treat SAD patients with phototherapy, having them sit in front of bright fluorescent lights at specific times of the day, usually early in the morning. In some cases, they have also begun prescribing antidepressants and other drugs.

Unfortunately, much of the research on the effectiveness of light treatments has been flawed; a review of 173 published studies found that only 20 had a proper design and suitable controls (Golden et al., 2005). But a meta-analysis of the data from those 20 studies did throw some light on the subject, so to speak. When people with SAD were exposed to either a brief period (e.g., 30 minutes) of bright light after waking or to light that slowly became brighter, simulating the dawn, their symptoms were in fact reduced. Light therapy even helped people with mild to moderate non seasonal depression (Pail et al., 2011).

SAD may occur in people whose circadian rhythms are out of sync; in essence, they have a chronic form of jet lag (Lewy et al., 2006). Or they may also have some abnormality in the way they produce or respond to melatonin (Wehr et al., 2001). They may produce too much daytime melatonin in the winter, or their morning levels may not fall as quickly as other people's. However, it is not clear why light therapy also appears to help some people with nonseasonal depression. True cases of SAD may have a biological basis, but if so, the mechanism remains uncertain. Keep in mind, too, that for many people who get the winter blues, the reason could be that they hate cold weather, are physically inactive, do not get outside much, or feel lonely during the winter holidays.

Does the Menstrual Cycle Affect Moods? Controversy has persisted about another long-term rhythm, the female menstrual cycle, which occurs, on average, every 28 days. During the first half of this cycle, an increase in the hormone estrogen causes the lining of the uterus to thicken in preparation for a possible pregnancy. At midcycle, the ovaries release a mature egg, or ovum. Afterward, the ovarian sac that contained the egg begins to produce progesterone, which helps prepare the uterine lining to receive the egg. Then, if conception does not occur, estrogen and progesterone levels fall, the uterine lining sloughs off as the menstrual flow, and the cycle begins again. The interesting question for psychologists is whether these physical changes cause emotional or intellectual changes, as folklore and tradition would have us believe.

Most people nowadays seem to think so. They are often surprised to learn that it was not until the 1970s that a vague cluster of physical and emotional symptoms associated with the days preceding menstruation—including fatigue, headache, irritability, and depression—was packaged together and given a label: premenstrual syndrome (PMS) (Parlee, 1994). Since then, most laypeople, doctors, and psychiatrists have assumed, uncritically, that many women “suffer” from PMS or from its supposedly more extreme and debilitating version, “premenstrual dysphoric disorder” (PMDD). What does the evidence actually show?