Introduction to Psychology 1/IPSY103/Consciousness/Biological rhythms

Figure 1. Biological clock affects the daily rhythm of many physiological processes. This diagram depicts the circadian patterns typical of someone who rises early in morning, eats lunch around noon, and sleeps at night (10 p.m.). Although circadian rhythms tend to be synchronized with cycles of light and dark, other factors such as ambient temperature, meal times, stress and exercise can influence the timing as well.

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. For example, body temperature fluctuates cyclically over a 24-hour period (Figure 1). Alertness is associated with higher body temperatures, and sleepiness with lower body temperatures.

This pattern of temperature fluctuation, which repeats every day, is one example of a circadian rhythm. A circadian rhythm is a biological rhythm that takes place over a period of about 24 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. If we have biological rhythms, then is there some sort of biological clock? 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.

Figure 2 shows that light affects the suprachiasmatic nucleus, a neural centre situated in the hypothalamus. The SCN regulates many different bodily functions including sleep, alertness, hormones, body temperature, digestion, and immune functioning.

The brain’s clock mechanism is located in an area of the hypothalamus known as the suprachiasmatic nucleus (SCN) (Figure 2). 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)

Reflection question

Are you a "morning person" or a "night person"? Do you regularly feel sleepy at a certain hour of the day? Do you need a certain number of hours of sleep to function well? Answering these questions demonstrates that our alertness/sleepiness shows consistent circadian patterns.