The body’s internal clock doesn’t just dictate when we sleep — it also determines how quickly our muscles heal. It appears muscle injuries heal faster when they occur during the body’s natural waking hours.
In relation to this, a new science study has produced findings that could improve understanding of ageing and obesity. The research finds muscle injuries heal faster when they occur during natural wake periods. Moreover, the findings shed light on the impact of shift work, jetlag and daylight saving time change.
As a follow-on, the Northwestern Medicine study identifies key immune and metabolic mechanisms linked to the circadian clock (the body’s intrinsic time-keeping mechanism). For the study, researchers performed single-cell sequencing of injured and uninjured muscles in mice at different times of the day.
Time of day affects your inflammatory response
They found that the time of day influenced inflammatory response levels in stem cells, which signal to neutrophils — the “first responder” innate immune cells in muscle regeneration. This showed that the activity of cells’ signalling to each other was much stronger right after injury when mice were injured during their wake period.
The scientists also established that the muscle stem cell clock also affected the post-injury production of the signalling metabolite nicotinamide adenine dinucleotide (NAD+). This is a coenzyme found in all cells that is essential to creating energy in the body and is involved in hundreds of metabolic processes.
Next, using a genetically manipulated mouse model, which boosted NAD+ production specifically in muscle stem cells, the team of scientists found that NAD+ induces inflammatory responses and neutrophil recruitment, promoting muscle regeneration.
Health risks of shift work
These findings could have implications for shift workers and may also prove useful in understanding the effects of aging and obesity. The study also may help explain how disruptions like jetlag and daylight saving time changes impact circadian rhythms and muscle recovery.
According to lead researcher Clara Peek, assistant professor of biochemistry: “In each of our cells, we have genes that form the molecular circadian clock. These clock genes encode a set of transcription factors that regulate many processes throughout the body and align them with the appropriate time of day. Things like sleep/wake behaviour, metabolism, body temperature and hormones — all these are circadian.”
The findings are additionally relevant to understanding the circadian rhythm disruptions that occur in aging and obesity. Circadian disruptions linked to aging and metabolic syndromes like obesity and diabetes are also associated with diminished muscle regeneration.
The research appears in the journal Science Advances titled “Immunomodulatory role of the stem cell circadian clock in muscle repair.”
