In the comments of an Al Fin post on the reduction of sleep pressure in mice through regulation of a brain chemical, I wondered if the diurnal circadian rhythms of most primates, including humans, might have an evolutionary protective effect.
The thought process is straightforward: To wander around fruitlessly in the dark, at risk of walking unexpectantly into a predator's ambush, seems like a wasteful and dangerous way to expend energy. Better to be quiet and still when the sun's not out. Conversely, konking out during the day makes for a sitting duck missing valuable hunting and gathering time.
Another commenter dismissed the idea, arguing that sleep serves other functions. Right, if the only purpose is protection--or "preservation and protection", to be exact--why sleep at all? Diurnal mammals are not disciplined enough to stay quiet and still during the night, perhaps. But why then the rebound sleep the following day?
If, however, it optimizes the time in which necessary functions take place, that objection doesn't seem convincing. There are lots of anecdotal evidence that for many people, adjusting to night shift work is never a completed process, even after having burned the midnight oil for years. If I go to bed without an alarm at eight in the evening, I will often sleep until five or six the next morning. If I'm up until four in the morning, I'll crash exhaustedly, yet still wake up around eight or nine if I made some attempt to darken the room. If not, I'll wake up with the rising sun a few hours later.
Rebound sleep is deeper than normal sleep, so more time is spent in REM during the late morning after 24 hours of being awake than would be the case if sleeping normally, but I assume that's a result of deprivation. The sleep would be deeper still after 36 hours of being awake during the following evening after having gone the night before and all of the current day without it. It's been my experience that night workers are often able to sleep for long periods of times during the nights on their days off (my roommate does this) while most people who work normal workweeks are not easily able to sleep undisturbed from nine in the morning until five in the afternoon on a Saturday.
Night work lowers serotonin transmission and night workers experience more disruption in sleep quality from caffeine than daytime workers do. Night workers average less sleep during the workweek than daytime workers do, and sleep deprivation is linked to lots to harmful things like inflammation and more negative emotiveness. Why the recalcitrant desire to sleep most during the night instead of whenever it is most convenient? Is it all cultural?
I realize that uninformed speculation on possible evolutionary effects makes serious geneticists cringe. That's why I'm bringing it here (with much humility!), since I have some allies in that arena who might have something to say.