Exercise and immune rhythm

A single session of moderate-intensity exercise lasting 30 to 60 minutes produces a significant acute increase in the circulation of virus-fighting immune cells and white blood cells that attack bacteria, the immune cells most relevant to viral and bacterial defence. These cells are mobilised from storage sites in the spleen, lymph nodes and bone marrow into circulation by the stress hormones released during exercise. In the two hours following exercise, immune cell counts return toward baseline, but the sensitised immune cells that have been mobilised and activated remain functional and distributed throughout the body. Over time, regular moderate exercise is linked with stronger immune resilience and lower chronic low-grade inflammatory load. The immune benefits of a consistent exercise habit are cumulative and well-documented across multiple study types.

What is happening

The long-term immune effect of regular exercise is different from the short-term mobilisation that happens during a single hard session. Repeated exercise sessions reduce the amount of visceral fat, which is metabolically active and a major source of pro-immune alarm signals. Exercise also influences immune signalling molecules involved in inflammatory balance. In the long term, consistently active individuals have lower circulating levels of inflammation markers including C-reactive protein and an inflammation marker than sedentary individuals matched for age, weight and diet. This is one reason physically active populations tend to show better long-term immune and metabolic profiles.

The immune-suppressing effect of excessive training is often called the open window hypothesis. After prolonged intense exercise lasting more than 90 minutes, circulating immune cell counts drop below baseline for several hours, creating a window of increased susceptibility to infection. Cortisol rises substantially during prolonged intense exercise and remains elevated during recovery, directly suppressing immune cell activity. Glutamine, which immune cells use as a primary fuel source, is depleted by muscle protein breakdown during heavy training. Elite endurance athletes have documented rates of upper respiratory infection two to three times higher than moderately active controls, particularly during heavy training blocks and in the days immediately following major competition.

What to do next

For hospitality workers, touring professionals and frequent travellers who may be physically active on their feet for long shifts but not systematically training, the relevant question is whether their physical activity load, combined with sleep deprivation and stress, has crossed into the territory where it is net immunosuppressive. The markers are fatigue that does not resolve with normal sleep, recurrent minor infections, reduced motivation and prolonged recovery from illness. These are signs that the total physical and psychological load has exceeded the recovery capacity available, and immune function is declining as a consequence. Reducing intensity, improving nutrition and protecting sleep are more effective responses than pushing through, because the immune suppression is biological rather than motivational.

Sources: WHO physical activity guidance, NHS exercise guidance, WHO stress guidance, NCBI sleep stages.

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