Sunlight spans the electromagnetic spectrum, but the red and near-infrared (R/NIR) spectrum is of particular interest for its biological effects. This spectrum includes visible red light (630 to 700 nanometers) and invisible near-infrared light (up to about 1400 nanometers). The sun is the most powerful natural source of this bioactive light, which can penetrate human tissue and influence cellular processes. Seeking solar red light aims to harness these longer wavelengths for potential benefits while managing exposure to other parts of the spectrum.
Understanding the Red Light Spectrum
The mechanism that creates the visible red and orange hues during certain times of day is called Rayleigh scattering, which explains how light interacts with the Earth’s atmosphere. Sunlight is composed of all visible colors, each traveling in a different wavelength, with blue and violet light having shorter wavelengths and red and orange having longer ones. As sunlight enters the atmosphere, tiny gas molecules like nitrogen and oxygen scatter the shorter-wavelength blue light in all directions, which is why the sky appears blue during the day.
When the sun is low on the horizon, its light travels a significantly greater distance through the atmosphere. This extended pathway causes the majority of the short-wavelength light (blues and greens) to be scattered away before reaching the observer. What remains is the less-scattered, longer-wavelength red, orange, and near-infrared light, which passes more directly through the dense atmospheric layer. This filtering process maximizes the concentration of R/NIR light reaching the surface when the sun is at its lowest angle.
Identifying Peak Solar Red Light Windows
The optimal periods for maximizing natural red light exposure are during the first hour after sunrise and the final hour before sunset. These windows are when the sun’s angle is lowest, providing the highest ratio of R/NIR light relative to UV radiation. Unobscured sunlight during these times is estimated to deliver a power density of R/NIR light that is comparable to levels used in many research studies.
Direct skin exposure is necessary to receive the full benefits, as clothing and glass can significantly block these wavelengths. A brief exposure of approximately 5 to 20 minutes is suggested, though the duration varies based on location, season, and individual skin sensitivity. Seek the warm, visible light on the skin without allowing discomfort or the onset of burning. Monitor the duration carefully to avoid negative effects.
Navigating UV Exposure and Safety
While the risk of sunburn from UVB radiation is reduced when the sun is low, ultraviolet A (UVA) light remains present. UVA rays penetrate deeper into the skin and contribute to long-term damage and aging. Therefore, caution is warranted with any sun exposure, even when the sun appears red or orange.
Eye protection is particularly important during low-angle sun exposure, as the sun is closer to the horizontal line of sight. UV exposure to the eyes can sometimes be higher in the early morning and late afternoon than at midday. Never look directly at the sun, even during sunrise or sunset, and consider wearing UV-protective sunglasses to shield the eyes. Limiting the duration of exposure and remaining attentive to the body’s response mitigates potential risks while experiencing the R/NIR benefits.