Period and Cryptochrome: The Genes That Run Your Body's Clock

In 2017, the Nobel Prize in Physiology or Medicine went to three scientists -- Jeffrey Hall, Michael Rosbash, and Michael Young -- for something they discovered decades earlier in ordinary fruit flies: a gene they named period that builds up in a cell overnight and breaks down during the day, on an almost exactly 24-hour cycle. That single gene, and a partner gene called cryptochrome, turned out to be the molecular gears inside a clock that ticks in nearly every cell of your body.

- How the PER and CRY proteins create a 24-hour feedback loop inside cells - What happens when a mutation speeds up or slows down that clock - Why chronobiology research earned a Nobel Prize in 2017 - How clock genes connect to real health effects like shift work and jet lag

Inside your cells, the period (PER) and cryptochrome (CRY) genes are switched on in the morning and start building up their proteins throughout the day. By evening, enough PER and CRY protein has accumulated that the two proteins pair up and move into the cell's nucleus. Once inside the nucleus, the PER-CRY pair does something clever: it shuts off the very genes that made it in the first place. Protein levels fall overnight, which eventually turns the genes back on again the next morning, and the whole cycle restarts. This rise-and-fall loop takes almost exactly 24 hours to complete, which is why scientists call it a molecular clock, and it's running inside cells in your liver, skin, and brain -- not just a single 'master clock' organ.

In 2001, geneticist Ying-Hui Fu identified families with a rare condition called Familial Advanced Sleep Phase Syndrome (FASPS), caused by a mutation in a clock gene. People with FASPS feel overwhelmingly sleepy around 7:30 p.m. and wake up wide awake around 4:30 a.m. -- their entire molecular clock is shifted several hours earlier than average, all traced back to a single gene mutation changing how quickly the PER protein builds up and breaks down.

Researchers have confirmed the same PER/CRY system in mice by 'knocking out,' or disabling, the genes -- mice without working clock genes lose their normal sleep-wake rhythm entirely, even in constant darkness. In humans, cryptochrome-related proteins in the eye also help detect light, which is part of why bright blue light from screens at night can suppress the sleep hormone melatonin and confuse the body's clock. This research has led to a growing field called chronotherapy, where doctors time medications to match a patient's molecular clock -- for example, timing certain blood pressure or chemotherapy drugs to the hours when the body will respond to them most effectively.