Shift Work and Late Nights May Worsen Muscle Loss, Scientists Warn

Muscle loss in old age has long been written off as an unavoidable fact of life. But a new scientific review suggests that a surprisingly overlooked factor may be accelerating that decline: a broken internal clock.

Researchers from West China Hospital of Sichuan University have published a review arguing that disruptions to the body’s circadian rhythm, its roughly 24-hour internal timing system, play an important and underappreciated role in the development of sarcopenia, the age-related loss of muscle mass, strength, and physical function. Published in the Chinese Medical Journal, the review makes clear that this goes well beyond poor sleep. It describes a cascade of biological failures rippling through muscle cells, energy systems, and the immune response when the body’s internal clock falls out of sync.

Sarcopenia affects between 5% and 13% of adults aged 60 to 70, and that number climbs sharply among people over 80, reaching anywhere from 11% to 50%. Despite decades of research, no drugs approved by the U.S. Food and Drug Administration exist to treat or slow it. The authors argue that a key reason for this gap is an incomplete understanding of the underlying biology, and that the body clock deserves far more attention than it has received.

What Your Body Clock Actually Does to Your Muscles

Every cell in the human body contains a molecular clock, a set of genes that cycle through activity patterns on a roughly 24-hour loop. In muscle, these clock genes regulate some of the most fundamental processes that keep tissue healthy: how proteins are built and broken down, how cells generate energy, how insulin moves sugar out of the bloodstream, and how cells clear out their own damaged components through an internal recycling system.

When those clock genes malfunction or fall out of rhythm, the consequences for muscle are real. Genes responsible for building muscle protein show altered activity. Proteins that break down muscle tissue lose their normal day-night patterns when the clock is disrupted. In mice engineered to lack functioning clock genes, researchers observed visible reductions in the number of energy-producing structures inside muscle cells, along with declines in how efficiently those structures could fuel muscle activity.

Blood sugar handling offers another example of the clock’s reach into muscle. Skeletal muscle clears roughly 60% to 80% of blood sugar after a meal. When key clock genes are reduced in muscle tissue, cells become less sensitive to insulin, meaning sugar stays in the bloodstream instead of being used for energy. Studies found that this clock-driven insulin resistance is mediated by a protein called SIRT1.

Shift Work, Late Nights, and Muscle Loss

Some of the most telling evidence in the review comes from human population data. A large South Korean study cited in the review found that shift workers had a 1.7-fold higher prevalence of sarcopenia compared to people who had never worked shifts, with irregular shift schedules showing an even stronger association than regularly scheduled ones.

Sleep timing matters too. Research cited in the review found that people who tend to stay up late, sometimes called evening types, faced independently higher risks of metabolic problems and sarcopenia compared to morning-oriented people. Even setting aside age, circadian disruption itself appears to raise the risk of muscle decline. A systematic analysis concluded that sleep quality may predict sarcopenia risk, and studies have found a U-shaped relationship between sleep duration and muscle health: both too little and too much sleep are linked to worse outcomes.

Artificial light at night is also flagged as a contributor. Animal experiments showed that constant or mistimed light exposure can trigger insulin resistance and disrupt the normal rhythmic activity of muscle clock genes. A mouse model exposed to prolonged light pollution developed insulin resistance in skeletal muscle, tied to elevated levels of a circulating molecule that interferes with insulin signaling.

Inflammation’s Role in Muscle Loss

Circadian disruption does not damage muscle in isolation. It interacts with another key driver of muscle loss: chronic, low-grade inflammation throughout the body, sometimes called inflammaging. This is not the acute inflammation that comes with an injury or infection. It is a persistent, smoldering state of immune activation that quietly degrades tissue over years.

According to the review, the relationship between the clock and inflammation runs in both directions and feeds itself. Disrupted clock genes appear to amplify inflammatory signals. When certain clock proteins are absent or reduced, inflammatory molecule levels rise. At the same time, those inflammatory signals suppress clock gene activity, creating a damaging loop where inflammation breaks the clock and a broken clock drives more inflammation.

Aging fat tissue becomes infiltrated with pro-inflammatory immune cells, which worsen this body-wide inflammation. That inflamed fat can migrate into skeletal muscle, further driving dysfunction and tissue breakdown.

Could Resetting the Body Clock Treat Muscle Loss?

Existing anti-inflammatory drugs have not proven particularly effective against sarcopenia. The review notes that even canakinumab, an anti-inflammatory drug tested over an eight-week period in people with a muscle-wasting disease called sporadic inclusion body myositis, showed no improvement in grip force or total muscle strength. Blocking inflammation alone, the authors argue, is not enough.

Instead, the review proposes a two-part strategy: restoring circadian rhythm function while simultaneously resolving inflammation through a class of naturally produced compounds, derived from certain fatty acids including omega-3s, that help the body actively end inflammatory states rather than simply suppress them.

On the circadian side, the researchers point to several promising directions. One class of compounds, working by activating a clock protein that plays a key regulatory role in muscle, has been shown in animal studies to improve energy production in muscle cells and reduce the proteins responsible for muscle breakdown. Exercise timing also emerges as a meaningful tool. Studies found that a single session of aerobic exercise significantly increased the activity of a key clock gene in trained men in the hours following the workout. In studies cited in the review, morning exercise appeared to shift the body clock earlier, while muscle performance tended to peak later in the day.

Diet is part of the picture as well. Caloric restriction, time-restricted eating, and specific plant compounds like resveratrol have been shown in various studies to influence clock gene activity in muscle and reduce muscle loss in animal models. Omega-3 fatty acids, long studied for their anti-inflammatory properties, also appear to affect clock gene expression.

Wang and Hong are careful about the limits of the current evidence. Much of the mechanistic detail comes from animal studies and lab models. Human population studies consistently link circadian disruption to worse muscle outcomes, but those studies are observational and cannot prove cause and effect. Individual factors like sleep preferences, existing health conditions, and the practical difficulty of sticking to structured timing routines all complicate how well circadian-based interventions might work in real elderly patients.

Compounds that target the clock directly are still in early stages of clinical investigation, with limited human data on their safety and effectiveness in aging populations. The authors call for large-scale, well-controlled clinical trials as the essential next step, including trials that might combine timed exercise with anti-inflammatory nutritional strategies like omega-3 supplementation.

Muscle decline in old age involves more than cells wearing out. Part of the problem is timing, and the evidence gathered here points to a real possibility: getting the body’s internal clock back on track may eventually give researchers another way to tackle age-related muscle loss.

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