red light and circadian skin science: a research review by our expert Professor Gaby Badre

PBM works when red or near-infrared light is absorbed by molecules inside the mitochondria, the parts of the cell that generate energy. This can increase the production of ATP, the molecule that provides usable energy for cellular activity. This can trigger further cellular responses, including signals that regulate cell activity and changes in nitric oxide, a molecule involved in blood flow, inflammation, and tissue repair.

Downstream, these effects activate signalling pathways that promote collagen synthesis, tissue repair, and growth-factor production, while reducing enzymes involved in collagen breakdown.

A key principle of PBM is the biphasic dose-response relationship, too little light may be ineffective, the right amount can be beneficial, and too much may reduce the positive effect. For superficial cutaneous applications, the therapeutic window generally lies between 8 and 20 J/cm².

The evidence for red and near-infrared photobiomodulation is no longer based on a few isolated reports. Recent reviews and clinical studies suggest that this approach can reduce inflammation, support tissue repair, and help skin recovery, while appearing generally safe when used within accepted treatment ranges. At the same time, the field is not yet fully standardised. Studies often use different wavelengths, doses, treatment schedules, and devices, which makes direct comparison difficult.

The strongest clinical support at present concerns wound healing and repair-related conditions. Analyses of controlled trials suggest that photobiomodulation can improve wound healing and reduce pain, although study quality and methodology remain uneven. There is also growing support for skin rejuvenation and anti-aging applications, with controlled studies using red light, often around 630 to 660 nm, reporting improvements in facial appearance and signs of skin renewal after repeated sessions.

Other applications appear promising. Hair loss, including female hair loss and alopecia areata, has shown moderate to strong responses in some studies. Chronic wounds may benefit especially when red and near-infrared wavelengths are combined, acne may improve through anti-inflammatory effects, and scar prevention and post-procedural healing are also areas of interest.

Safety remains a central concern. Reviews examining multiple cell types have not found evidence that clinically relevant red-light treatment promotes malignant change. This does not remove the need for caution in specific medical situations, but it supports the overall safety profile of PBM when properly used.

The skin functions as a peripheral circadian organ, with a local timing system that regulates repair, renewal, barrier recovery, immune activity, and protection against stress. These processes do not remain constant over 24 hours. Cell renewal, DNA repair, oxidative defence, temperature regulation, sebaceous activity, and inflammatory responses all vary according to the time of day.

Molecular studies confirm this rhythmic activity. Skin biopsies taken at different times show that a significant proportion of skin genes are expressed according to a circadian pattern, particularly those involved in DNA repair, cell division, and barrier maintenance. In skin that is chronically exposed to light, this organisation becomes disturbed, fewer genes maintain a clear rhythm, and the timing of their activity shifts.

The skin is also directly sensitive to light. It contains photoreceptors, including neuropsin (OPN5), melanopsin, and cryptochromes, which allow it to respond locally to light. Experimental studies show that skin tissue can detect changes between day and night and adapt to a shifted light-dark cycle.

Blue light appears particularly relevant because it can activate melanopsin and alter circadian gene activity. When these rhythms are disrupted, protective functions such as barrier integrity and repair may also be affected. Circadian disruption, whether from irregular light exposure, sleep loss, or shift work, may weaken the skin’s ability to recover and increase its susceptibility to environmental damage.

These findings have clear implications for photobiomodulation. Red-light PBM appears to influence many of the same pathways that are under circadian control, including mitochondrial function, oxidative balance, inflammation, and tissue repair. This makes it plausible that treatment response depends not only on wavelength and dose, but also on when the skin receives the light.

Most research on light and biological rhythms has focused on blue light because of its strong effect on the brain’s central clock. Red light appears to act differently. Early studies suggest that it may support circadian physiology in a gentler way, particularly in relation to sleep.

In some studies of whole-body red or near-infrared photobiomodulation, participants reported improved sleep quality, along with higher melatonin levels and lower nighttime heart rate. These effects appeared even when no clear improvements were seen in physical performance, suggesting that red light may influence sleep regulation more than exercise outcomes.

Its effects may also depend on timing. During the day, PBM appears to support cellular energy production and brain oxygenation. During sleep, it may enhance the brain’s glymphatic system, which is more active at night and linked to circadian regulation.

In the skin, red light influences oxidative balance, inflammatory signalling, calcium regulation, and collagen production. It supports both surface renewal and deeper structural repair, while helping to regulate inflammation in damaged tissue. Some studies suggest a possible indirect effect on melatonin, although the mechanism remains uncertain.

From a practical perspective, timing may become an important factor in treatment design. A session delivered when the skin is naturally more prepared for repair could be more effective than the same treatment at another time of day. Red light around 660 nm does not significantly activate retinal melanopsin, making it less likely to interfere with melatonin and sleep, and more compatible with evening use.

Red-light photobiomodulation is increasingly supported as a safe and promising non-thermal approach in dermatology. Current evidence suggests benefits for skin repair, wound healing, scar support, and skin rejuvenation. At the same time, the skin is now understood as a light-sensitive organ with its own daily biological rhythms, making treatment timing a potentially important factor.

Even so, the field remains at an early stage. Much of the research has focused either on the disruptive effects of blue light or on general PBM mechanisms, with less known about how red light interacts with the skin’s own circadian system. Many studies are limited by small sample sizes, short follow-up, and variability in treatment protocols.

Red-light PBM appears to have genuine value, particularly in repair and rejuvenation contexts. Circadian timing is a promising variable, but not yet an established therapeutic principle.