The Secret Power of Daylight That Supercharges Your Immune System

A new discovery by researchers at the University of Auckland reveals that natural daylight plays a surprising and powerful role in strengthening the immune system. The study focused on neutrophils, the body’s most common type of white blood cell, and found that these cells become better at fighting infections during the day—thanks to an internal biological clock that responds to light.

This exciting finding could lead to new light-based therapies aimed at helping the body defend itself against infections and inflammatory diseases.

Neutrophils Have Internal Clocks

Neutrophils are our body’s first line of defense. When bacteria enter the body, these fast-moving immune cells are the first to attack. But not all neutrophils are equally effective at all times of day.

Using transparent zebrafish to observe live immune activity, scientists discovered that neutrophils carry a built-in circadian clock—a kind of biological timer that responds to daylight. These clocks enhance the cells’ ability to produce reactive oxygen species (ROS), which are chemicals that destroy invading bacteria.

Researchers observed that neutrophils become most active during daylight hours, matching the body’s natural active phase. This synchronization is thought to be an evolutionary advantage: when we’re awake and moving, we’re more likely to come into contact with bacteria, so it makes sense for the immune system to be extra alert during this time.

The Role of Light in Circadian Immunity

How light and immune system function are connected

Light is the key signal that resets circadian clocks in most cells, including those in neutrophils. In this study, scientists found that exposure to light activated specific genes in the neutrophils, especially one called hmgb1a. This gene helps produce the molecules needed to kill bacteria. Without light, or when the gene was mutated, the neutrophils were far less effective at fighting infections.

This discovery not only explains why our immune response is stronger during the day, but also opens the door to light-based medical strategies. By targeting the neutrophil clock, doctors may one day be able to enhance immunity for patients with chronic inflammation or recurring infections.

What This Means for Future Therapies

The ability to "tune" the immune system using light or clock-targeting drugs could transform the way we treat diseases. Imagine adjusting hospital lighting or developing medications that enhance the light-responsive clock inside neutrophils. These approaches could be especially helpful in critical care, infection recovery, or even for people with autoimmune diseases.

As more research unfolds, the intersection between light therapy and immune health could become a core area of interventional psychiatry and holistic medicine.

Discover more at interventionalpsychiatry.org

References:

1. Hall, C. et al. (2025). A light-regulated circadian timer optimizes neutrophil bactericidal activity to boost daytime immunity. Science Immunology. https://www.science.org/doi/10.1126/sciimmunol.adi5001 

2. Neuroscience News. (2025, May 26). Daylight Boosts Immune Defense. https://neurosciencenews.com/daylight-immune-system-29144/