According to a new study in the journal Science Translational Medicine, injuries you sustain at night can take much longer to heal. Your body’s circadian rhythms make the healing process slower to start off at nighttime, which delays recovery for weeks after you got hurt.
Scientists at the Medical Research Council Laboratory of Molecular Biology in Cambridge, England, analyzed the medical records of 118 burn victims between the ages of 18 and 65 in England and Wales. People burned between 8 AM and 8 PM healed 60 percent faster than people who were injured at night. That translates to average of 28 days for nighttime burns versus 17 days for day burns—a huge difference.
Based on the study, the cells involved with repairing tissue damage seem to get locked into a less active state when they begin the healing process at night. “The analogy that comes to mind is two sprinters doing the 100 meters,” says study author John O’Neill, a research group leader at the Laboratory of Molecular Biology. “The sprinter that is wide awake and poised, waiting in the blocks for the starting pistol will never be caught up by the sprinter who just got woken up in the middle of the night and is still in their pajamas.”
Circadian rhythms affect your body temperature, how you sleep, your appetite, and a wide range of other bodily systems. Over the course of roughly 24 hours, certain genes in your body are either more active or less active, which results in higher and lower levels of certain proteins involved with biological functions at specific points in the day.
Previous research had determined that one of the systems dependent on your body clock included fibroblasts, which are precursor cells that form structures and grow new tissue during healing. But no one had yet examined how circadian rhythms might affect healing in a measurable way.
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In a series of experiments in cell cultures, the scientists identified several proteins involved with the production of actin, a component of the cell’s cytoskeleton structure, which were cranked down during inactive hours in mammals. “The activity of the cytoskeleton is essential for allowing cells to move, like a cell's muscles really,” O’Neill says. “Therefore, skin’s cells move more quickly to repair wounds during the active phase, compared with the rest phase when humans would normally be sleeping.” Still, it’s not yet clear why your body can’t simply catch up on repair during the daytime when you’re injured the night before.
The researchers then studied how mouse skin tissue transplanted onto a membrane responded to punctures and scratches. The tissue maintains its circadian rhythms even after it’s been removed from the original host. (Don’t worry—they didn’t stab live mice for this study.) By examining the injured skin samples at different time intervals, the team identified visibly fewer amounts of fibroblasts near the wound sites when the tissue was injured during what would have been rest time for the mice. As a result, a single off-hours puncture would still be festering after more than two days, while a wound from the mouse’s active hours would close after less than two days.
That finding lined up with the analysis of healing times in daytime and nighttime burn victims. Since fibroblasts are found in tissues throughout the body, the researchers expect that cuts, scrapes, and potentially more serious injuries you suffer during nighttime would all be affected by slower healing times.
Aside from accidental injuries, the results of this study could even inform the best times of day for people undergoing surgery or other invasive medical procedures. “The thing to consider is whether people who are early risers should be scheduled for morning operations, and people who are later risers should have them in the afternoon,” O’Neill says. For emergency surgeries that aren’t scheduled, it’s possible that using drugs to reset cellular clocks could help speed healing. But we don’t know yet. “More human studies would be required before any change to clinical practice should be considered, of course.”
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