It's both, Jim. Retina-to-brain chemistry and skin-to-vitamin D synthesis.

Retina-wise, it's the missing blue light natural light has in abundance that has been missing for so long in incandescent lighting. Being well in yellow light is like eating yellow snow. Sure that will give some relief if one is dying of thirst, but it's not as good as clean, clear water! :~)

So it's not just the brightness (specs and descriptions are upthread), but also the color rendering of the light.

~S~