I remember reading somewhere that it is analogous to closing your eyes under the sun, as your eyelids only allow some wavelengths of sunlight. Not sure if that's accurate.
The transmittance of light through the human eyelid is about two orders of magnitude greater at the start of the near-IR (~700) nm than it is at 450nm. [1]
[1]
"Measuring and predicting eyelid spectral transmittance"
Journal of Biomedical Optics, Vol. 16, Issue 6, 067011 (June 2011) https://doi.org/10.1117/1.3593151
It's quite analogous, especially for ideal intensity
Bright enough to warm the body, but not thermally stress the cells.
And yeah, closing your eyelids would block UV light from getting in and cross linking proteins and causing cataracts, while still allowing for NIR light to get in and speed up the mitochondria.
Not exactly a scientific experiment, but based on this video it seems like fire would have significantly less visible intensity for it to make a meaningful difference.
Weirdly they didn't test nighttime exposure as an intervention. OK, "weird" means probably it was costly/hard to get people into a lab setting at 9pm... But it does make you wonder!
A different study found that red light helps only if it precedes exposure to blue light, blue light and UV light being the main sources of (oxidative) stress on the retina. Knowing that is probably why they didn't bother testing night-time exposure.
Some believe that the reason red light helps is that cytochrome C oxidase can use the red light to convert serotonin to melatonin and that melatonin is the most important anti-oxidant in the human body. (Under this model, melatonin cannot get into the mitochondria, but serotonin can.)
A few decades ago I had this mysterious sunny morning with colors 10 times brighter than usual. I never managed to figure it out, now the only things I remember are that the curtains were open, the sun came in, I had a dark red box next to my bed that I must have looked at for 20-30 seconds because the color was so absurdly vivid, then I looked at a gold lamp on top and thought woah! The effect persisted for a while, everything in my house looked different. I've also noticed changes in my vision when sleeping in different places.
What little I remember it sure fits your description. I might try hooking up a red heat lamp to a timer as an alarm clock to see what happens.
I bought an infrared parabolic heater to test this out once, as a friend made a comment insinuating that more time in front of fire meant a better sleep. I ran it for a few days, but the results were mixed and inconclusive.
I suspect because the longer IR wavelengths just heat the body up, it's hard to get enough of the more "useful" wavelengths without overheating
It has UV, which stresses out the cells
You can't PWM the sun, it's hard to get full naked coverage in front of it. Doesn't perfectly target NIR or NIR-II biological windows.
