Immunity in the light: How daylight boosts the body's defences

While most of us associate sunlight with good moods and vitamins, scientists from New Zealand have made an important discovery: daylight directly affects our body's ability to fight infections.

Researchers from the University of Auckland have found that immune cells are more active and more effective in destroying bacteria during the day. This was made possible by studying the behaviour of neutrophils - the most numerous white blood cells - in a model animal: the transparent fish danio rerio.

Details: Lucia Du et al, A light-regulated circadian timer optimises neutrophil bactericidal activity to boost daytime immunity, Science Immunology (2025). DOI: 10.1126/sciimmunol.adn3080. www.science.org/doi/10.1126/sciimmunol.adn3080

Light as a signal to attack

Neutrophils are a kind of "special forces" of the immune system. They are the first to come to the rescue in case of inflammation or bacterial invasion. As the study showed, these cells have an inbuilt biological clock sensitive to light. It is light signals, rather than the time on the clock, that set the rhythm of their activity.

Doctoral students at the University of Auckland together with Professor Christopher Hall found that neutrophils kill bacteria much faster and more efficiently during the day than at night. Light literally "energises" the immune system, turning on the defence mode at full capacity.

This was possible thanks to a unique feature of danio fish - their bodies are transparent, which makes it possible to observe the behaviour of cells in a living organism with high precision. In this way, the researchers saw how immunity works depending on the time of day.

Immunity on a schedule

Our bodies obey circadian rhythms, an internal clock that governs sleep, metabolism, heart function and now, as it turns out, immune system activity. Light serves as the main external factor that synchronises these biological processes.

This discovery could revolutionise approaches to treating inflammatory diseases and infections. If it is possible to learn how to control the internal clock of neutrophils, it will be possible to develop new methods of strengthening immunity without unnecessary stress on the body. For example, to administer drugs at the time of day when the cells are most active, or to develop drugs that mimic the light signal.

What's next?

Researchers are now focused on understanding the precise molecular mechanisms by which light affects the circadian rhythms of neutrophils. Understanding these processes will pave the way for a new generation of drugs that can "wake up" immune cells and boost their response at the right time - for example, in the morning, when the risk of infection is higher.

The findings were published in the journal Science Immunology. This is a step towards thinking of immunity not as a non-stop defence, but as a finely tuned system that responds to environmental signals - primarily light.