Sleep is not a luxury; it is an absolute necessity for our mammalian brains to function properly. It is a stretch to call it subjective. While it may be loosely thought of as subjective in the sense that we are capable of staying awake for varying periods of time, it is not something that we are capable of healthily carrying on without. The times of day/night that we need to sleep are pretty consistently and inherently set in our brain, and the amount that is optimal has been extensively studied.
The simple fact is that our brain does a lot of work throughout a day, and, in its standard operation, waste materials build up that, in large enough quantities, cause dysfunction or, when pushed to extremes, damage of the brain. During sleep, the space around our neurons and glial cells expands, and, as I understand it, the postulated function of this is to permit better flow of cerebrospinal fluid through our brains and thus flush them of waste material. I had mammalian sleep explained to me this way by an Anatomy and Physiology teacher - quite simply, sleep is not something that our brains can go without, and a properly functional brain (that is properly producing melatonin) sets and enforces a pretty strict circadian rhythm wherein we are awake during the day and asleep during the night. That is not to say that we are not capable of flexibility in this regard, but pushing our limits in regards to sleep is not a healthy practice.
Natural selection tends towards favoring the individuals in a population that display traits that make them superior survivors and reproducers within the population's environment - while you say that it would tend towards individuals who were capable of maintaining proper function without sleep, I would argue that sleep is a sort of built-in necessity of our complex and highly developed brains and that proper function of our brains can not be maintained without it. I would hypothesize that any individuals that were born with genetic traits that incapacitated or limited their ability to sleep (i.e., genetically induced insomnia) would be at a pretty extreme disadvantage compared to other members in the population. Although there are many nocturnal predators, none have been enough of a threat to our species that it has limited us in any significant or tangible way, and nor have any of them become so well adapted that they regularly threaten or prey upon us while we sleep.
Natural selection does not do much in the way of acting on neutral traits - neutral genetic traits tend to remain relatively consistent in a population as long as the environment remains relatively consistent such that, relative to the environment, those traits do not become particularly advantageous or disadvantageous. While there are exceptions wherein extremes are favored, it seems clear that any disadvantages posed by sleeping at night (in regards both to our modern species and our ancestors) are far outweighed by the advantages of being fully alert and functioning during our regular operational hours.