What Are Circadian Rhythms?
Circadian rhythms are the physical, mental, and behavioral changes an
organism experiences over a 24-hour cycle. Light and dark have the biggest influence on circadian rhythms, but food intake, stress, physical activity, social environment, and temperature also affect them. Most living things have circadian rhythms, including animals, plants, and
microorganisms. In humans, nearly every
organ has its own circadian rhythm, and collectively they are tuned to the daily cycle of day and night.
Circadian rhythms influence important functions in the human body, such as:
- Sleep patterns
- Appetite and digestion
What Scientists Know About How Circadian Rhythms Are Controlled
The system that regulates an organism’s innate sense of time and controls circadian rhythms is called a
biological clock. It’s composed of
proteins encoded by thousands of
genes that switch on and off in a specific order. A master clock coordinates all the biological clocks in an organism.
vertebrate animals, including humans, the master clock exists in the brain. The human master clock is a large group of
nerve cells that form a structure called the suprachiasmatic nucleus (SCN). Among other functions, the SCN controls production of the hormone melatonin based on the amount of light the eyes receive. In the evening, a person’s master clock tells their brain to make more melatonin, causing sleepiness. The SCN also synchronizes the circadian rhythms in different organs and tissues across the body.
In 2017, NIGMS-funded researchers Jeffrey C. Hall, Michael Rosbash, and Michael W. Young won the
Nobel Prize for their circadian rhythms research. They identified a protein in fruit flies that has a role in controlling normal daily biological rhythms. During the daytime, this protein (called PER) is produced by the cell but immediately broken down in the
cytoplasm, keeping PER protein levels low. When night falls, a protein called TIM binds directly to PER, protecting it from breaking down. The PER-TIM complexes enter the
nucleus and stop the cell from making additional PER. Then, as day breaks, the PER-TIM complexes break down, the block on PER
transcription is lifted, and the cycle repeats.
In this way, PER regulates its own synthesis through a negative feedback loop. Feedback loops are coordinated systems that link the output of the system to its input. For example, a thermostat functions on a feedback loop: A home’s furnace will turn off when the house reaches the set temperature and only turn back on when the temperature falls below that threshold again. In the case of PER, the protein directly controls the transcription of the gene that codes for it.