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Disruptions to a person's normal sleep cycle, such as pulling several all-nighters, could lead to an increase in harmful triglycerides in the blood, a new study on mice suggests.

Though further studies are needed to firm up whether the same holds for humans, scientists often use these rodents as models for human systems.

The new findings could have implications for understanding the health effects of night shifts, 14-hour work days and transoceanic flights. High levels of triglycerides in the blood are a risk factor for heart disease and obesity – not only in mice, but also potentially in people, the researchers say.

Light-dark cycle

Most of Earth’s organisms, from bacteria to humans, live according to a natural clock called a circadian rhythm, tied to Earth’s 24-hour day and night cycle. This clock is set by our genes, but can be affected by external factors such as daylight and temperature. We’re mostly active during the day and inactive at night, and our metabolism reflects that schedule.

For instance, energy-storing fat molecules in the blood called triglycerides are highest during the day, when we need the extra energy lift.

Mice, on the other hand are nocturnal – they feed at night and sleep during the day. As a result, the level of triglycerides is high at night and low during the day.

Various external features, such as keeping bright lights on in the wee hours (for us), can throw circadian rhythms out of whack.

"We know that if mice are kept in the dark for a long time, their bodies lose the rhythm," said Mahmood Hussain, a professor of cell biology at the State University of New York’s Downstate Medical Center, in New York City. He led a team of scientists in exploring the ways that the circadian rhythm regulates triglyceride levels in mice.

Sleep disruptions

To understand how the circadian rhythm affects triglyercide levels, the researchers examined a protein called CLOCK, which is known to play a role in controlling the circadian rhythm, in mice.

They found that at dawn (when the rodents are getting shut-eye) CLOCK proteins triggered several reactions to occur within cells that resulted in lower levels of triglycerides in the blood of normal mice.

The researchers also tested the triglyceride levels of mice that had a mutation in the gene for CLOCK, rendering it ineffective. They found that because the CLOCK gene did not "activate" at daybreak, it couldn’t trigger the normal series of reactions, and the levels of triglycerides in the blood stayed high, even during the day while the mice were supposed to be sleeping.

Sleep and fat

The results suggest disruptions to daily rhythms in humans could also affect levels of triglyceride levels, or other aspects of our metabolism.

"We’ve completely trained ourselves out of the normal day-night cycle as it is," Hussain told Life's Little Mysteries. "We have lights so we can stay up, we work late, and we sleep in late on some mornings. Is this disruption harmful to us? It’s affecting our entire physiology and we should know how."

Hussain said that the team plans to next examine the consequences of this particular circadian rhythm disruption. He thinks that mice with high triglyceride levels as the result of a mutated gene for the CLOCK protein might be more susceptible to problems such as hardened arteries, obesity and xanthomatosis, a disease characterized by lumps of cholesterol deposits.

Hussain said that studies also are now being done on the impact of jet lag and transcontinental flights, which can sometimes cause headaches, insomnia and disorientation in passengers.

The study was published in the Aug. 4 issue of the journal Cell Metabolism.

 This article was provided by Life's Little Mysteries, a sister site to LiveScience.