In thinking about recent human genetic evolution, I've tried to wonder more about the brand new environment that pastoralists carved out for themselves, rather than looking only at how sedentary farming environments differed from our earlier hunter-gatherer ways. Most such discussion is about agriculture vs. hunting and gathering, but pastoralism is distinct from both and has been practiced at non-trivial levels for hundreds to thousands of years. Why not look there for signs of recent natural selection at work? (Of course the research on lactose tolerance in adults looks at pastoralists, but I mean changes in something other than digestion.)
Right now I'm just going on intuition, but I figure it's not so bad since I must be descended from fairly nomadic / pastoralist people, especially on my mother's Scotch-Irish hillbilly side. But I'm fact-checking on google just to be safe.
After going for a refreshing twilight / night-time stroll through green and hilly places, it occurred to me that a night owl schedule must be an adaptation to a pastoralist way of life, especially for those tribes that practice transhumance, where the livestock themselves are mobile rather than penned in, and where the young males tag along to tend to them. If you think about it, this is the group that stands the most to gain by staying awake, alert, and energetic during the night, and the least to lose by sleeping in or being awake yet drowsy during early bird hours.
First, the benefits. Farmers don't get any work done during the night, so all they would stand to gain is the ability to protect the persons, animals, and material stuff on their property from raiders, thieves, etc. However, since farmers are sedentary, they can scare a lot of these types off by building defenses -- fences or wires around their animals and crops, houses to protect the people and material possessions, and a place to store a variety of weapons to chase off intruders. Plus they live in large groups, so each individual can "slack off" and rely on at least one other person waking up in response to danger. Hunter-gatherers don't have much stuff worth stealing in the first place, so they would only benefit by not being surprised in a pre-dawn raid by a neighboring tribe. Again, they can rely on strength in numbers and slack off in guard duties since at least one person will wake up.
Nomadic pastoralists, however, do have plenty of stuff worth stealing -- their flock -- and yet cannot build stable defenses around it, as it doesn't stay put. They also have to worry much more about non-human threats like nocturnal predators (such as leopards and hyenas in Africa) picking off members of their flock. And a particular flock tends not to be looked after by a large band, so vigilance is more of an individual responsibility. Lastly, some livestock are not even asleep but engage in "night grazing" when they find the heat unbearable during the daytime. Here the flock could just wander off if you weren't awake to pay attention! Shepherds tending to such breeds do in fact stay awake during the night. For all these reasons, it would pay a lot to be a night owl in a nomadic pastoralist society compared to a farming or hunting and gathering society.
Then, the costs. You figure humans have a fixed amount of time or energy that they can be awake, so there's a trade-off between a daytime or night-time schedule. Night owls would have less time to spend working during the day, and for the same stretch of time they'd be more tired than early birds during the day. Farmers would lose big-time by sleeping in or being drowsy during the morning, given how essential sunlight is for planting seeds, threshing wheat, etc. Hunters and gatherers too would lose big-time, given how crucial sunlight is to see what you're hunting / tracking, as well as to identify safe-to-eat nuts, berries, tubers, etc. by visual signs such as color and shape.
Pastoralists don't stand to lose as much, though. They might have to be awake to watch over their flock, but they could stand being lethargic during the day. After all, most of the threats they face would prefer not expose themselves in broad daylight -- roaming thieves, animal predators (except for the lion), etc. Compared to farmers and hunter-gatherers, pastoralists can get by more easily on autopilot during the daytime, and so don't face as high of a cost by lounging around -- hence the ubiquity in pastoral poems of laying in the shade and playing the pipes.
With more to gain and less to lose by moving toward a more night owl schedule, pastoralists are the natural targets of genetic changes that would shift circadian rhythms toward such a schedule.
As a rough check, here is a list of traits found more in night owls vs. early birds. It sure sounds like night owls are more nomadic in their personality and behavior -- more impulsive, less conscientious, less agreeable (i.e., more confrontational), more independent and non-conformist, and more extravagant (use google images to compare flamboyant pastoral costume to drab farmer wear, e.g. the pastoralist Maasai vs. Bantu farmers).
So count this as another gift from pastoralist culture -- nightlife.