Sleep: Learn It 1—Biological Rhythms

  • Explain circadian rhythms and how they can be disrupted
  • Describe the brain structures and hormones involved in sleep
  • Describe theories about the function of sleep
  • Understand the difference between REM and non-REM sleep
  • Describe the stages of sleep
  • Compare and contrast theories about why we dream

Biological Rhythms

Biological rhythms

Biological rhythms are internal rhythms of biological activity. There are several types of biological rhythms, including:

  • circadian rhythms: a biological rhythm that takes place over a period of about 24 hours. For example, changes in body temperature and alertness that fluctuate cyclically over a 24-hour period (Figure 1). Alertness is associated with higher body temperatures, and sleepiness with lower body temperatures.
  • infradian rhythms: a biological rhythm that lasts longer than 24 hours. A woman’s menstrual cycle is an example of an infradian rhythm—a recurring, cyclical pattern of bodily changes. One complete menstrual cycle takes about 28 days, or a lunar month.
  • ultradian rhythms: a biological rhythm that lasts less than 24 hours. For example, the rhythm of the body’s appetite.
A line graph is titled “Circadian Change in Body Temperature (Source: Waterhouse et al., 2012).” The y-axis, is labeled “temperature (degrees Fahrenheit),” ranges from 97.2 to 99.3. The x-axis, which is labeled “time,” begins at 12:00 A.M. and ends at 4:00 A.M. the following day. The subjects slept from 12:00 A.M. until 8:00 A.M. during which time their average body temperatures dropped from around 98.8 degrees at midnight to 97.6 degrees at 4:00 A.M. and then gradually rose back to nearly the same starting temperature by 8:00 A.M. The average body temperature fluctuated slightly throughout the day with an upward tilt, until the next sleep cycle where the temperature again dropped.
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, 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 to our state of consciousness. Circadian and other rhythms are regulated by a variety of biological clocks within an organism. In humans, as with other vertebrate animals, these biological clocks are coordinated by a “master clock” known as the suprachiasmatic nucleus (SCN).

biological clock

A biological clock, or an innate timing device, is comprised of specific molecules (proteins) that interact in cells throughout the body. Research indicates that humans (as well as other animals, insects, fungi, and plants) have biological clocks.

Some form of a biological clock is found in nearly every tissue and organ. In the brain, the hypothalamus, which lies above the pituitary gland, is a main center of homeostasis. Homeostasis is the self-regulating tendency of biological systems to maintain internal stability within a changing external environment.[1] In humans, the brain’s main 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. 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).

In this graphic, the outline of a person’s head facing left is situated to the right of a picture of the sun, which is labeled ”light” with an arrow pointing to a location in the brain where light input is processed. Inside the head is an illustration of a brain with the following parts’ locations identified: Suprachiasmatic nucleus (SCN), Hypothalamus, Pituitary gland, Pineal gland, and Output rhythms: Physiology and Behavior.
Figure 2. The suprachiasmatic nucleus (SCN) serves as the brain’s main clock mechanism. The SCN sets itself with light information received through the retina of the eye.

Problems with Circadian Rhythms

Generally, 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 external cues, such as light, atmospheric conditions, temperature, and social interactions, to set their biological clocks.

Chronotypes

There exist individual differences with 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.

  1. Billman, G. E. (2020). Homeostasis: The Underappreciated and Far Too Often Ignored Central Organizing Principle of Physiology. Frontiers in Physiology, 11. https://doi.org/10.3389/fphys.2020.00200 ↵