Washington ? Like kids taking apart a fine Swiss watch, scientists are laboring to understand what makes the biological clock that’s inside every living creature tick.

Researchers have long known that bacteria, flies, worms, flowers, oak trees and human beings all have tiny internal timepieces that keep them on a roughly 24-hour cycle, the time it takes the Earth to spin once on its axis.

“Living cells can actually tell the time and use this information to control their behavior,” said Hugh Nimmo, a plant biologist at the University of Glasgow, Scotland.

Many questions remain to be answered, however, such as how the clocks work at the level of individual molecules. To find out, some scientists are building simple biological clocks in a test tube.

“If you can build it, you really understand it,” said Jonathan Arnold, a geneticist at the University of Georgia in Athens. “It’s very important that we know how the clock works at the molecular level.”

These living pacemakers keep running whether it’s light or dark. Creatures that live underground or at the bottom of the ocean continue to have timers that they inherited from their ancestors, even though they no longer see the sun shine.

Inner clocks tell people when to wake up and plants when to unfurl their leaves. For small nocturnal mammals, knowing when dawn is coming can mean life or death from a predator. Contrariwise, a fungus may wait to send up its reproductive stalks until after dark to avoid dangerous ultraviolet light.

When clocks go awry, they contribute to miseries such as insomnia, liver disease and cancer, Arnold said. In humans, a gene that the biological clock controls is involved with early-morning heart attacks.

Organic timing mechanisms are governed by one or more clock genes in a cell’s DNA.

The genes produce specialized proteins — long strings of organic molecules — that control the sequence of bodily functions.

Taken together, the genes and proteins make up a complex regulatory network that fits together like the gears in a watch. As in a clock, the timing can be reset to compensate for daylight saving time, night work, jet lag and even a slightly longer day-night cycle on Mars, if humans ever land there.

People’s clock genes may set their sleep patterns. Last summer, Sarah Forbes-Robertson, a British researcher at the Swansea University School of Medicine in Wales, reported that she can tell whether a person is an early riser or a night owl by inspecting a gene called REV-ERB in his or her DNA, taken from a swab on the cheek.

A low level of gene activity is associated with sleep, a high level with wakefulness, she said.

“If your peak is earlier than 4 p.m. it would indicate that you are a natural early bird,” she said. “If you peak later than 5 p.m., then you are more of a night owl.”