Sleeping in cover, if just a rock outcropping or under some brush, is much safer than sleeping out in the open. Maybe that’s what happens, your mind maintains extra readiness at some cost.

Bingo. If there's light predators can see you. If it's pitch black you're in a cave or burrow.

Away from cities it gets very dark at night, unless there is a full moon. But near cities there is a lot of light pollution, so we're always in a state of near-twilight.

Celestial lights are quite bright when away from light pollution

That's not very true. Usually yes, but sometimes moonlight is actually very light and I find it pretty comparable to city lights. In cities, you don't realize how bright moon can get, and it's clearly not bright every single night, but about 2 nights every month it gets pretty bright.

Our ancestor's sleep might not have been optimal due to the fact we had other things to care about, like shelter. But that does not imply there is nothing to gain from feeling perfectly rested.

Yeah it's sometimes hard to understand that evolution is not a perfection machine, it's not survival of the fittest, and it does not select the "best" outcome and discard the second best. It's survival of the least unfit, and it selects any option that doesn't cause the individual to die before reproducing. Just because our ancestors did something and nature didn't punish them with death because of it does not mean it's the optimal way to go about life. It merely means that doing that thing didn't cause our ancestor to die before their kids were born. Evolution doesn't select for perfection, just reproduction.

Same argument when someone says "when I was a kid we did X and we lived". Doesn't mean sleeping on the back dashboard of the car on the freeway is a good idea, just that you in particular did not die from doing so.

The point of that argument is never "This is perfect." It's exactly as you say - "This is not bad enough to cause serious harm and therefore" (they're saying) "not bad enough to get worked up over it."

"It survived evolution" = "It won't kill you" = "It's good enough"

They're setting the bar a bit lower than optimum.

My understanding is that different wavelengths effect sleep hormones differently. My less than educated guess would be that evolutionarily speaking there are different optimal sleep environment cycles based on wavelength, intensity, and temperature variables depending on location and habits of ancestors.

You're absolutely right, our body most definitely reacts in different ways to different wavelengths of light.

A specific green has been shown to help reduce pain. [1] 525 nm seems to be the magic wavelength in this particular case.

I'm a chronic acute pain sufferer. I'd love for something as simple as an array of green LEDs to help with my pain. I keep meaning to get a large breadboard, a bunch of LEDs, and make such a device.

I wonder what effects other wavelengths might have? Would lights help sleep if they matched sunset darkening to night time through purple and indigo? It's fascinating stuff.

https://www.ncbi.nlm.nih.gov/pubmed/28092651

I have an amibilight built around WS2812 LED strip and esp8266. I use it to display a colorful rotating gradient during a day and as a night light with red color.

Small Haskell program using reactive-banana-automation feeds simple nodemcu firmware every second with UDP packets. It's quite easy to build RGB light this way as only one signal wire is required.

Red light helps cells heal [1], ultraviolet light can kill bacteria. I'll find sources and measurements.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4148276/

There was an interesting piece [1] on the BBC a couple of days ago about research finding exposure to Cyan might help keep us awake.

[1] https://www.bbc.co.uk/news/education-44565320

Also would assume ancestors go to sleep with the sun and wake up with the sun. Modern humans rarely do.