The visual perception of light is an important input for managing one’s circadian rhythm. While most of us in the developed world seem to get more light than we should later in the evening than we should, people with complete blindness have the opposite problem: not seeing any light really screws up their sleep/wake cycle.
Somewhere in between those two extremes is a happy medium. What is it? How many hours of light per day are our bodies expecting in order to maintain a healthy 24-hour cycle with best sleep quality/quantity? If humanity got its start in the rift valley (very close to the equator), then it seems like something close to 12 hours from sunrise to sunset is appropriate, since that’s what that region gets year-round. Even in the northernmost reaches of the Scandinavian peninsula, 12 hours is probably right for the pasty white folk who evolved into that niche; it’s pretty much the average of summer daylight and winter daylight.
That’s my best guess, anyway. What does science say?