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Biological Rhythm: Definition & Explanation

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  • 0:02 Biological Rhythm Definition
  • 0:39 Circadian Rhythm
  • 1:45 Menstrual Cycle
  • 2:35 Other Examples
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

Learn what biological rhythms are and how they fit into the life cycles of different animals. Read about the circadian rhythm and other examples of biological rhythm that cause behaviors and growth.

Definition of Biological Rhythm

Let's say it is 3:00 in the afternoon. Today you woke up, ate some food, walked around, and maybe did some other stuff. Later you will eat some more food and then go to sleep. Does any of this mean anything? Yes, you have biological rhythms that influence what you do.

Biological rhythms are cyclical activities that are a basic characteristic of animal life. These are behaviors or developments that occur as part of a life cycle.

Here is an image of a cycle; notice the circular pattern of beginning, middle, end, and then the process is repeated. Each follows an internal biological clock which starts and stops certain processes.

biological clock

Circadian Rhythm

One of the most common biological rhythms is the circadian rhythm, which roughly translates to about (circa-) day (-dian). The circadian rhythm is a rest-activity cycle that is centered on light, meaning when a preset amount of light occurs, an animal will be active, and at another time, the animal will rest. Humans are active when there is a lot of light, which is usually during the day, and rest when there is less light, usually at night. Humans have developed ways around this by having artificial lights which keep the rest part of the cycle at bay. Cats are active during the night, meaning they are more active when there is less light and rest during the day when there is more light.

However, the amount of light only influences the circadian rhythm; it does not set it completely. For example, if you kept a person locked in a dark room they would have periods of activity. This is because there is a free-running rhythm in animals that activates the animal's brain and says, 'Get up! Time to get up.' This rhythm can also be affected by movements. The problem of jet lag occurs when the free-running rhythm is set to a different time.

Menstrual Cycle

menstrual cycle chart

Another example of a biological rhythm is the female menstrual cycle. This image shows us as estrogen (the gray line) increases, follicle stimulating hormone or FSH (the blue line) decreases. Once the estrogen hits a certain level, this causes the body to react and release large amounts of luteinizing hormone, or LH (the pink line), and extra FSH. This extra LH and FSH cause estrogen to start dropping. When estrogen starts to drop, progesterone (the purple line) climbs before dropping. Then, the whole cycle starts over.

This complex example with four different hormones is an example of a longer-term biological rhythm. It is a cycle that occurs regularly in most women unless there is something that interrupts or interferes with it, such as pregnancy, stress, or a chemical.

Other Examples of Biological Rhythm

Lets look at some other examples.

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