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Night time injuries heal more slowly

People who are injured at night heal far more slowly because the body’s internal clock regulates the pace of healing and performs better during the day, researchers said Wednesday.

The report published in the journal Science Translational Medicine found that cuts and burns healed about 60 percent faster if the injury happened during the day compared to at night.

Researchers say the body’s clock is the reason for the difference, because it regulates crucial body processes including sleeping, metabolism and hormone secretion.

Skin cells that help patch up wounds work more quickly in the daytime than they do at night, thanks to the workings of our circadian clock. The finding suggests patients might recover from injury more quickly if they have surgery during the right time of day.

“This is the first time that the circadian clock within individual skin cells has been shown to determine how effectively they respond to injuries,” said author John O’Neill at the Medical Research Council (MRC) Laboratory in Cambridge.

“We consistently see about a two-fold difference in wound healing speed between the body clock’s day and night,” he added.

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“It may be that our bodies have evolved to heal fastest during the day when injuries are more likely to occur.”

The study was based on experiments using both live mice and human skin cells in a lab dish, and was corroborated with records of 118 burn patients from major burn units in England and Wales.

To find out, John O’Neill, a biologist at the Medical Research Council’s Laboratory of Molecular Biology in Cambridge, UK, and his team studied a type of skin cell known as fibroblasts that are essential for wound healing. Fibroblasts invade the void left by a scratch and lay the foundation for new skin to grow.

The cells are also known to keep their own time. For example, cultured cells exhibit rhythmic oscillations in gene expression where there is no input from the master clock.

The researchers then tested that hypothesis with cells grown in a flat layer in a petri dish. The fibroblasts filled in scratches more quickly during the day than at night.

“You can see by eye, when the cell is wounded only 8 hours apart from each other, in a different circadian phase, the daytime wounded ones take off, and the nighttime one drags,” O’Neill says.

The researchers then showed in mice that skin wounds suffered during their waking hours healed better than ones incurred during their resting hours. What’s more, those increases lined up with the cell culture data. About twice as many fibroblasts migrated into the daytime wounds as nighttime ones. “We were really astonished,” O’Neill says.

Nighttime burns — happening between 8 pm and 8 am — took an average of 60 percent longer to heal. Burns incurred at night were 95 percent healed after an average of 28 days, researchers said in report.

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Daytime burns healed in just 17 days, on average, because skin cells moved to the site of the wound to repair it with proteins like actin and collagen much faster during the day.

The same process was apparent in mice and human cells in a lab dish, suggesting that the body’s internal circadian clock is in charge of this process.

“Further research into the link between body clocks and wound healing may help us to develop drugs that prevent defective wound healing or even help us to improve surgery outcomes,” said lead author Ned Hoyle, also from the MRC Laboratory of Molecular Biology.

With Inputs from Agency