Why natural light?

We have evolved with light that constantly changes both in intensity and spectral composition, through cycles on several levels (like days, lunar months, or years). It always comes from a black-body radiator: some hot material which emits light according to its temperature. This also includes wood fire, which does not affect our sleep negatively. 

In the past decade, much research has been done to discover how various frequencies of the spectrum have specific physiological effects. These effects often act in different directions but through overlapping mechanisms — therefore, they are also in interaction with each other. We still know little about the details and we are far from having a complete map of the spectrum regarding the effects of every frequency band.

On the one hand, this means that light can be used to evoke a desired biological reaction in therapeutic applications (e.g. photobiomodulation: using red and near-infrared light for tissue healing).

On the other hand, electric light with highly unnatural spectra can disturb the physiological balance that our body expects from our light environment. Therefore, the safest option to preserve health is as much natural light as possible. 

The role of long-wavelength light in tissue recovery (photobiomodulation)

Near-infrared light has pervasive and important effects through local melatonin production:

Zimmerman & Reiter (2019) Melatonin and the optics of the human body. Melatonin Research, 2.

Photobiomodulation can protect the retina from oxidative stress, with implications for macular degeneration:

Kim & Won (2022) Effect of Photobiomodulation in suppression of oxidative stress on retinal pigment epithelium. International Journal of Molecular Sciences, 23.

A summary on the many benefits of sunlight through photobiomodulation, including how red and near-infrared light slows down tissue aging:

Heiskanen, Pfiffner, & Partonen (2020) Sunlight and health: Shifting the focus from vitamin D3 to photobiomodulation by red and near-infrared light. Ageing Research Reviews, 61.

A short summary on the importance of long-wavelength light in general lighting:

Veto (2020) Correspondence: Daylighting: Why infra-red should be explored. Lighting Research & Technology, 52.

Ongoing study at Pacific Northwest National Laboratory on the cognitive, mood, and cardiovascular effects of long-wavelength light:

Roddick (2022) Near Infrared Effects on Cognitive Performance, Mood, & Cardiovascular Physiology.

Short summary on NASA projects in photobiomodulation, including for eye health:


Different frequency — different effect (in various applications)

Age-related macular degeneration protection index: nothing beats natural light (see Planckian radiators, Figure 4):

Schierz (2019) Is light with lack of red spectral components a risk factor for age-related macular degeneration (AMD). CIE Proceedings of the 29th CIE Session.