Quirks & Quarks

If you're going to get hurt, you're better off doing it during the day

Daytime wounds heal twice as fast as at night, scientists find.
There's a lot of other factors that go into the severity of our injuries and how quickly we heal. (Shane MacKichan/CBC)

Time heals, but could it be faster?

Wounds heal 60 per cent faster in the day than at night, scientists have found. 

Researchers in the U.K. made the discovery about how time heals injuries mainly by studying mice.

Dr. Ned Hoyle is the lead author of a study published in this week's issue of the journal Science Translational Medicine.

Hoyle, a postdoctoral fellow at the MRC Laboratory of Molecular Biology in Cambridge, U.K., Dr. John O'Neill and their team looked at how long wounds take to heal over 24-hour periods. 

On the left, cells heal over a wound more quickly during the day than on in the night, as seen on the right. 0:11

The researchers took small biopsy samples and surgical cuts that didn't harm the mice. The samples allowed them to look at how a type of skin cell, called fibroblasts, move into and across wounds to repair the damage. 

A scar forms

Fibroblasts are workhorses in much of the body. Their job is to secrete the matrix that cells live in and on. In wound healing, that's particularly important because there's a big hole to fix. By excreting extracellular matrix, a scar forms.

Hoyle calls fibroblasts a model for the circadian or body clock instead of using tissues or whole mice. 

The main finding is that the speed at which wounds heal can depend on the time of day when a wound is inflicted. Wounds inflicted in the "active" phase are quicker to heal. For most humans, the active phase is during the day. In mice it's at night because they are nocturnal.

They studied how long it took to achieve 95-per-cent healing. In humans, the effect size at night was 60 per cent — the equivalent of 11 more days to heal for wounds at night than in the day.

Hoyle says he was shocked at the large effect. The investigators got historical clinical records and could predict how long the movement of fibroblasts into the wound area would take, but they didn't expect that there would be such a stark difference by time of day in humans. 

Trick skin cells

The caveat is that it's very early days. Eventually Hoyle would like to see if doctors could tailor treatments to early riser chronotypes (also called morning larks) when scheduling surgery or later for those with later chronotypes (owls). 

Further down the road, the researchers imagine applying a topical compound like a cream that tricks cells in to thinking it's daytime when it is really night. Scientists can already do this in a lab dish. If a cream could be placed on wounds when emergency surgery has to be done at night, for instance, it's possible that injuries won't take as long to heal.

John O'Neill's main research interest is circadian rhythms: the innate biological 24-hour clock that sustains daily cycles in behaviour and physiology in organisms as diverse as humans, plants and bacteria. (St. John's College/University of Cambridge)

Does this mean if you get into a fist fight at midnight, will you be in worse shape than if it happens at high noon? 


But there are a lot of other factors that go into the severity of our injuries and how quickly we heal, doctors say. 

Dr. Marc Jeschke is the medical director of Sunnybrook Health Sciences burn clinic in Toronto. It is the largest regional trauma centre in Canada. 

Jeschke says the patients he tends to see at night are often drunk or impaired by other substances. In contrast, those who come during the day are mainly patients with workplace accidents where alcohol or drugs aren't involved. 

He says substance use itself could be delaying wound healing, so he recommends caution in interpreting Hoyle's findings. 

Ideas for future of medicine

Still, Jeschke welcomes the idea of someday trying to trick the skin of patients with night-time injuries into thinking it's daytime to speed up wound healing.

An expert in the field of circadian rhythms called Hoyle's experiment elegantly designed, using a variety of technologies from cells in a lab dish to live animal models.

Frank Scheer, director of the medical chronobiology program at Brigham and Women's Hospital in Boston, says it is the first human comparative data to investigate the role of the circadian system in wound repair and healing. 

Scheer says it's important to consider what might happen in shift workers or people who have circadian rhythm sleep disorders or jet lag when skin cells receive conflicting cues about the time. 

When scientists use clock mutant mice in their experiments to knock out the circadian system in skin cells, they've found evidence of harmful effects, Scheer says. It hasn't yet been studied in humans.