Nearly every tissue and organ in a human being runs on a twenty-four hour cycle, and those cycles stay in step with each other and with the day outside. Light and dark matter most, though food, stress, activity and temperature all push on them. Something has to coordinate the whole set. The surprise is what that coordinator is made of.
Proteins encoded by thousands of genes, switching on and off in a set order. A master clock synchronizes the rest, and in vertebrates it sits in the brain: a large group of nerve cells called the . It controls melatonin production according to how much light the eyes receive, and in the evening it tells the brain to make more, which is what makes a person sleepy.
Circadian rhythms are the physical, mental and behavioral changes an organism runs through over a twenty-four hour cycle. Light and dark influence them most, with food intake, stress, physical activity, social environment and temperature also acting on them. Most living things have them, animals, plants and microorganisms alike, and in humans nearly every tissue and organ has its own, collectively tuned to day and night.
The system that holds an organism’s innate sense of time is called a biological clock, and it is made of proteins encoded by thousands of genes that switch on and off in a specific order. A master clock coordinates all the others. In vertebrates it is the suprachiasmatic nucleus, a large group of nerve cells that sets melatonin production according to the light the eyes receive and keeps the body’s separate rhythms in step.
The molecular detail came from flies. Jeffrey Hall, Michael Rosbash and Michael Young won the Nobel Prize in two thousand and seventeen for identifying , a protein with a role in controlling daily rhythms. During the day the cell produces PER and breaks it down at once, keeping levels low. At nightfall a protein called TIM binds to PER and protects it, the complexes enter the nucleus, and they stop the cell from making any more PER. As day breaks the complexes break down, the block lifts, and the cycle repeats.
That is a : PER directly controls transcription of the gene that codes for it. The arrangement is also easy to knock off kilter. Gene variants can affect the clock proteins. Neurological diseases such as Alzheimer’s disjoint the rhythm, travel across time zones and shove the sleep-wake cycle around, and light from screens at night can confuse the clock outright.
wat. The clock is not a part. It is a protein that shuts off its own gene, waits, and starts again, about once a day.
wat wat
Timekeeping here is not a mechanism separate from the body. It is a loop of proteins in cells, which is why light at the wrong hour can reach it so easily.
How do we know?
A fact sheet from the National Institute of General Medical Sciences, which funds much of this work. It sets out what the clock is made of, where the master clock sits, and what happens when the timing slips.
What is still unknown
A public fact sheet. It gives the established picture and the open questions the institute is funding, without the experiments or the quantities behind either.
Learn something?
Where to next
What shape does human sleep take when the nights are long?sleep
What does the brain do at night that it cannot get done by day?sleep
Who decides which of today’s memories you still have tomorrow?sleep