It turns out skin cells follow a circadian clock of their own, patching up wounds more quickly during the day.
For a report published in Science Translational Medicine, John O'Neill and colleagues studied fibroblasts, a type of skin cell critical to healing wounds. They function by invading the void a scratch left, laying the foundation on which new skin can grow.
In cultured cells, fibroblasts exhibited rhythmic movement in gene expression even when input from a master clock was not provided. This kind of time-keeping ability inspired the researchers to look for proteins within fibroblasts that move along to daily rhythms. They discovered that the proteins controlling the skin cell's actin-based skeleton are active during the day.
Fibroblasts Favor Daytime Work
Based on observations, these proteins instruct fibroblasts to seek out injuries, spurring them to action to start the healing process. According to the researchers, this suggests that when a wound occurs may have an effect on how quickly it will heal.
To test this hypothesis, the researchers used three models: fibroblasts grown flat in a petri dish, on mice, and on humans.
"You can see by eye, when the cell is wounded only 8 hours apart from each other ... the [daytime] wounded ones take off, and the [nighttime] one drags," said O' Neill, talking about the petri dish model.
They also noted that the mice healed better when their wounds occurred during waking hours compared to those acquired during resting hours. Additionally, twice as many of the skin cells migrated into wounds suffered during the day.
Looking for evidence of the same effect in humans, O' Neill and his team turned to the International Burn Injury Database. After analyzing the data, they found that nighttime burns required on average 11 more days to heal than their daytime counterparts.
Further research is required to confirm the fibroblasts' behavior but O'Neill speculated that if what they observed was proven to be real, one application would be in surgeries. Given people are set to recover better depending on their circadian rhythms, then surgical procedures may be scheduled appropriately earlier in the day for morning people and later to accommodate night owls to promote faster healing of wounds.
Skin Cell Function
The time-varying response from fibroblasts may be the result of evolutionary adaptation, noted the researchers. As people were likelier to sustain injuries during their waking hours, the body primed skin cells to respond more effectively during the day.
The circadian clock has long been thought to be present only in the brain, receiving signals from the eyes to function. However, other researches have shown that cells in various parts of the body, including the liver and lungs, follow their own schedule. However, it's still not clear how certain cells are capable of following their own 24-hour clocks while others require external reminders.