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Why Wounds Heal Faster During the Day Than at Night

A new study suggests that you should consider staying away from sharp objects at night

Did you get that injury during the night or day? It might be telling about how long it'll take to heal. (Counselling via Pixabay)
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Whether you slice your finger while cutting an apple or chopping veggies doesn’t seem to matter much—either way it’s a painful experience. But a new study suggests that when you suffer a wound may actually influence how quickly it heals. As Andy Coghlan at New Scientist reports, researchers have found that wounds sustained during the day heal twice as fast as those that occur at night.

Whenever you are injured, a type of skin cell known as fibroblasts, move into the region to pave the way for new cells to grow. Fibroblasts are known to "keep their own time," writes Roni Dengler at Science, periodically changing activity depending on the time of day. But the details of the process remained largely unclear.

To better understand these rhythms, Nathaniel Hoyle and his team at the Laboratory of Molecular Biology in Cambridge took a closer look at the fibroblasts. While studying how proteins produced by the cells vary throughout the day, they discovered that proteins important to healing were most abundant when the sun was up.

To test if the difference in activity had a impact on wound healing, the team turned to wounded cells on a Petri dish. They measured the rates of healing at different times of day and discovered that, indeed, wound healing happened faster during the daytime.

“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,” study leader John O’Neill tells Dengler at Science.

In fact, about 30 different genes that control the movement of fibroblasts are more active during the day than at night. The researchers then turned to mice to test the idea, discovering that, as expected, daytime wounds healed faster than nighttime injuries.

When they examined data from the International Burn Injury Database, which includes data about the time of injury, they found the same pattern: burns sustained at night took an average of 11 days longer to heal than daytime burns. The research appears in the journal Science Translational Medicine.

So what’s going on? Dengler explains that for many decades, researchers believed that the circadian clock, the body's master clock, which is located in the hypothalamus, was the only thing that received signals about daytime and nighttime through visual cues.

That clock determines circadian rhythms, which control things like sleeping and waking, digestion, hunger and the release of hormones. A team of researchers won this year’s Nobel Prize for figuring out just how that process occurs on the molecular level.

But in recent years, researchers have discovered other parts of the body have their own independent clocks, including lungs and liver cells. It turns out, fibroblasts keep their own time as well, though researchers aren't sure exactly how the cells synchronize with the outside world. The finding could lead to changes in the way medicine is practiced. 

“This research adds to the accumulating evidence that ‘time of day’ or ‘circadian rhythmicity’ matters in medicine,” Derk-Jan Dijk at the University of Surrey, not involved in the study, tells Coughlan. “The question is how we can make use of this knowledge, and whether it can change clinical practice and help patients.”

The results hint that it might be beneficial to schedule surgeries to correspond to an individual’s circadian clock. Or perhaps certain drugs can trick the fibroblasts into thinking it's daytime, leading to better wound healing. In the meantime, it’s best to put the knives away once the sun goes down, just to be safe.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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