A team of researchers at MIT and the University of California at San Diego (UCSD) has shown how cell division in a type of bacteria known as cyanobacteria is controlled by the same kind of circadian rhythms that govern human sleep patterns.

Previous studies have shown that even though cyanobacteria do not "sleep" in the same way that humans do, they cycle through active and resting periods on a 24-hour schedule. Cyanobacteria depend on sunlight for photosynthesis, so they are most active during the day.

The researchers demonstrated how the circadian clock regulates the bacteria’s rate of cell division (their method of reproduction) in single cells. "These cells have to keep dividing, and the circadian oscillator regulates when they divide," says Bernardo Pando, an MIT graduate student in physics and one of the lead authors of a paper describing the findings in the March 18 online edition of Science.

In multicellular animals, including humans, cell division is critical for renewal and repair, while out-of-control cell division leads to cancer, so "understanding how cells are dividing is really of fundamental importance," says Susan Golden, professor of molecular biology at UCSD and an author of the paper.

Cyanobacteria maintain their circadian rhythms even when isolated from the naturally occurring daily light-dark cycles of the sun, just as humans do. The researchers found that under conditions of moderate constant light, the cyanobacteria undergo cell division about once per day, and the divisions take place mostly at the midpoint of the 24-hour cycle.

To figure out how the cell division cycle is coupled to the circadian clock, the researchers sped up the cell cycle by boosting the intensity of light, enabling the cells to photosynthesize more, which increases the amount of energy available to them. The cells did start to divide more frequently, but in a pattern still linked to the circadian clock—they divided once a quarter of the way into the cycle, and again three-quarters into the cycle.

The team also showed that the cyanobacteria enter a resting phase about 19 hours into the circadian cycle, after which they will not divide until the next cycle begins.