Day cuts heal faster than night ones

Wounds such as cuts and burns suffered during the day heal about 60% faster than those incurred at night, a new study has discovered.

The research, published in the journal Science Translational Medicine, shows how our internal body clocks regulate wound healing by skin cells and optimise healing during the day.

This could have implications for medical procedures such as surgery and the development of drugs that improve wound healing, according scientists from Britain’s Medical Research Council Laboratory of Molecular Biology, in Cambridge.

Circadian rhythms — our body clock, the 24-hour internal mechanism that runs in the background of our brains — regulate nearly every cell in the body, driving processes such as sleeping, hormone secretion and metabolism. Researchers found that wounds to the skin suffered during the body clock’s “daytime” healed almost twice as efficiently as wounds incurred during the night.

The report’s senior author, John O’Neill, speculates that our bodies may have evolved to heal fastest during the day when injuries are more likely to occur.

The study examined healing actions in human and mouse fibroblasts, the most common cells of connective tissue, and keratinocytes, the predominant cell type in the epidermis. 

The researchers analysed the records of 118 patients with burn injuries, using a database from all main burns units in England and Wales.

Burns that happened at night took an average of 60% longer to heal than burns that occurred during the day. Night-time burns (incurred between 8pm to 8am) were classed as 95% healed after an average of 28 days, while those suffered between 8am and 8pm took only 17 days to reach the same state.  

“We can reset the tissue-healing response by tricking the cells into thinking it’s a different time of day – such as by turning the lights on at night and off at different times of day for the mice, or using body clock-altering drugs on cells in the lab,” O’Neill says. 

“It may be that healing time could be improved by resetting the cells’ clocks prior to surgery, perhaps by applying drugs that can reset the biological clock to the time of best healing in the operation site.”

A key reason for this faster healing is that skin cells move to the site of the wound to repair it much faster during the body clock’s daytime.

Within the cells, this is driven by the increased activity of proteins involved in cell movement and repair, especially the protein actin. Filaments of actin provide scaffolding and movement, acting like a muscle within the cell.

In daytime wounds, there is also more collagen — the main structural protein in skin — deposited at the wound site, which continues for up to two weeks after the wound occurred.

The researchers note that this process must be driven by the cells’ internal circadian clocks, not signals transmitted throughout the body, since human and mouse skin cells grown in a laboratory dish show the same effect.

The authors caution that more research is needed to understand the effects in humans, especially whether changes to current surgical practice would be beneficial.

This research is an important addition to a great deal of study into how our body clocks control vital bodily mechanisms. 

In a recent interview in the journal Genome Medicine, Joseph Bass, a leading researcher into the circadian clock system, discusses advances in the field of chronopharmacology.

“Certain drugs are metabolised differently at different times of day,” he writes, “and, therefore, it may be optimal when studying drug metabolism to test the levels of the drug at different times in the cycle and to take advantage of this information to adjust and manipulate drug dosing”.

Bass, director of the Centre for Diabetes and Metabolism and Chief of Endocrinology at Northwestern University, in Chicago, US, uses a train analogy: “There would be time in biochemical processes as there is in a central train station controlling clocks in the brain, and all the organs would have their own trains running. Thus, inside these tissues, time is something relative to the brain clock or to somebody standing inside the brain looking out.”