Biological Rhythms

Biological rhythms are internal rhythms of biological activity. A woman’s menstrual cycle is an example of a biological rhythm—a recurring, cyclical pattern of bodily changes. One complete menstrual cycle takes about 28 days—a lunar month—but many biological cycles are much shorter. Biological rhythms such as the menstrual cycle are called infradian rhythms because they last longer than 24 hours, and others that last less than 24 hours are called ultradian rhythms. Changes in body temperature and alertness that fluctuate cyclically over a 24-hour period (Figure 1) are examples of a circadian rhythm. A circadian rhythm is a biological rhythm that takes place over a period of about 24 hours. Alertness is associated with higher body temperatures, and sleepiness with lower body temperatures.

Figure 1. This chart illustrates the circadian change in body temperature over 28 hours in a group of eight young men. Body temperature rises throughout the waking day, peaking in the afternoon, and falls during sleep with the lowest point occurring during the very early morning hours.

Our sleep-wake cycle, which is linked to our environment’s natural light-dark cycle, is perhaps the most obvious example of a circadian rhythm, but we also have daily fluctuations in heart rate, blood pressure, blood sugar, and body temperature. Some circadian rhythms play a role in changes in our state of consciousness.

Research indicates that humans (as well as other animals and plants) have a biological clock, or an innate timing device, comprised of specific molecules (proteins) that interact in cells throughout the body. Biological clocks are found in nearly every tissue and organ. Researchers have identified similar genes in people, fruit flies, mice, fungi, and several other organisms that are responsible for making the clock’s components. In the brain, the hypothalamus, which lies above the pituitary gland, is a main center of homeostasis. Homeostasis is the tendency to maintain a balance, or optimal level, within a biological system. In people, the brain’s clock mechanism is located in an area of the hypothalamus known as the suprachiasmatic nucleus (SCN). The SCN is comprised of about 20,000 nerve cells. The axons of light-sensitive neurons in the retina provide information to the SCN based on the amount of light present, allowing this internal clock to be synchronized with the outside world (Klein, Moore, & Reppert, 1991; Welsh, Takahashi, & Kay, 2010) (Figure 2).

Figure 2. The suprachiasmatic nucleus (SCN) serves as the brain’s clock mechanism. The clock sets itself with light information received through projections from the retina.

Problems with Circadian Rhythms

Generally, and for most people, our circadian cycles are aligned with the outside world. For example, most people sleep during the night and are awake during the day. One important regulator of sleep-wake cycles is the hormone melatonin. The pineal gland, an endocrine structure located inside the brain that releases melatonin, is thought to be involved in the regulation of various biological rhythms and of the immune system during sleep (Hardeland, Pandi-Perumal, & Cardinali, 2006). Melatonin release is stimulated by darkness and inhibited by light. People rely on zeitgebers, or external cues, such as light, atmospheric conditions, temperature, and social interactions, to set the appropriate biological clock.

There are individual differences in regard to our sleep-wake cycle. For instance, some people would say they are morning people, while others would consider themselves to be night owls. These individual differences in circadian patterns of activity are known as a person’s chronotype. A person’s individual chronotype may show that a person has a greater propensity to sleep earlier and wake up earlier (a morning lark), or to stay up late and sleep in (a night owl). Morning larks and night owls differ with regard to sleep regulation (Taillard, Philip, Coste, Sagaspe, & Bioulac, 2003). Sleep regulation refers to the brain’s control of switching between sleep and wakefulness as well as coordinating this cycle with the outside world.