2.11 Biological Rhythm: Definition & Explanation
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.
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.
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.
Hibernation and migration are both examples of rhythms based on the time of the year. When the seasons change, it causes behavioral changes. Either the animal barricades itself away and enters a dormancy period, using very little energy and sleeping away times when there is little food outside, or it moves to another area. Most people remember that bears go into hibernation during the winter months. During this cold time there is little available food. Only when the temperature increases does the bear wake up and go in search of food.
Migration is actually much more complex, because the animal is relying on brain structures and possibly genetic mechanisms to guide it. Some birds travel to a location they have been before, moving between two places at the change of the seasons each year. Salmon must return to a place they have been once, the spawning pool. The strangest are the monarch butterflies. The monarch butterflies migrate south for the winter, lay eggs and die. The second generation is born and migrates north for summer, without having ever been there before. The second generation lays eggs and the third generation migrates south, due entirely to a biological mechanism encoded into their genetics.
Biological rhythms are cyclical activities that are a basic characteristic of animal life. In humans, we see the circadian rhythm, or the rest-activity cycle, controlled primarily by light. However, even without light the rest-activity cycle can self-stimulate. In females the menstrual cycle is internally modulated by several hormones but can also be affected by the environment. Other forms of biological rhythms include the hibernation and migration habits of creatures. Their biological rhythms are influenced by temperature changes but are ultimately guided by complex brain and genetic influences.