Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

Study Shows That Wounds Sustained in Daytime Heal Faster

Dear Doctor: Apparently, wounds that occur at night don't heal as well as those that occur in the daytime. Why would that be -- and what can people do if they're hurt at night?

Dear Reader: An intriguing study that looked at wounds and the rates at which they healed reinforces the idea that the circadian clock, the inner timekeeper in virtually every living thing, is even more complex than was realized. Considering the circadian clock in humans is already known to manage sleep-wake cycles, hormone production, brain wave activity, body temperature and mood, to name just a few of its functions, that's pretty impressive.

The circadian cycle is a period of approximately 24 hours. During that time, and keyed to the daily shift from light to dark and back again, the circadian clock influences rhythmic changes in both physiology and behavior. Disruption to these cycles, whether short- or long-term, is known to adversely affect health and well-being. Research has shown that the cycle of sleep and wakefulness in mammals is keyed to visible light. A specific region of the brain receives and interprets the visual input from the eyes and sends out the appropriate cues of whether to sleep or wake up.

But recent studies have surprised scientists by suggesting that certain parts of the body, including the liver and lungs, have circadian rhythms that are independent of patterns of dark and light, and of the brain center that interprets those patterns. All of which (finally!) leads us to your question. According to research published in the journal Science Translational Medicine, burns that were sustained during the day took about 17 days to heal. The same types of injuries that were sustained at night required an additional 11 days -- or 28 days total -- to heal.

The reason for this turns out to be the behavior of fibroblasts, which are among the first cells to rush to the rescue at the site of an injury. These take various forms, depending on their location within the body. But what fibroblasts have in common is the ability to generate a matrix of specialized proteins that help a wound to contract and heal. How quickly they get to work depends on whether it's day or night.

When researchers looked at cells grown in a petri dish, at wounds in mice and at data from the International Burn Injury Database, which includes the time that a patient was injured, they discovered that daytime wounds healed a startling 60 percent faster than the same types of wounds when sustained at night. Because fibroblasts obey their own circadian clocks, which are independent of a person's own master clock, it turns out that they work harder and more efficiently during the daytime hours.

At this time there's no known way to override the fibroblasts' circadian clock. When you become injured during the nighttime hours, the healing process will be slower. However, researchers are already looking into how these insights might be used to improve medical care -- surgical outcomes, for example -- and to explore the impact on other forms of treatment.

(Send your questions to, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)