When we speak of the evolution of sleep, there is the simple physiological function to evaluate, but we might consider as well the developing social and psychological aspects of the practice. Many mammals are social animals, meaning that some within a group may sleep while others remain awake and on guard, as needed. Now, others have noted that for top predators, there really is little reason to do anything but sleep except when hunting or mating or grooming or perhaps engaging in a little play. The element of fending off mating rivals is really simply an extended element of the mating process, geared toward the successful promotion of one's own genes. So sleep they do, for long a slothful periods, the lazy lion and the tired tiger quite famously. For smaller animals living lower on the food chain, who may come to end up as prey, sleep often takes place in seclusion, in warrens or covens, dens, thickets, holes in the ground, or under cover of camouflage. And who has not known a dog of the house whose habits ran towards lengthy naps?

An interesting take on this topic from another angle is the social evolution of sleeping habits and accoutrements amongst humans. Naturally, human sleep began where most mammals slept, on the ground or (where foliage permitted) on sufficiently sturdy boughs. Even apes are observed to occasionally bend and twist the thin branches of the tree into a suitable sort of nest for a comfortable rest. But amongst the first innovations of human technology, coming along with the invention of cloth itself, was the invention of bedding; of at least the use of a section of man-made material regularly and repeatedly used as a surface upon which to sleep. And, in seemingly short order after that, of thicker sections of material upon which to rest the head so that it would be at a level a few inches above the rest of the body. And then of additional sections of material to lay over the sleeping body. Man is, naturally, hardly unique in preferring to sleep on top of prepared ground and under some material to guard against the elements, with many species being observed to dig and scratch and thump at the ground on which they perchance to nap, so as to level it out and soften it up for their bed. And, those with borrowing and covening behaviours do more than protect critters from eaters, as they address the elements as well, especially so for creatures whose burrows snugly contour to their bodies.

But the human predilection has, over the centuries, led to thicker mattresses, fluffier pillows, finer thread counts, more comfortable comforters, and the ornate four-poster canopy bed with headboard and night-tables. Indeed, since ancient times, one of the focal axes of the home has been the bedroom (how oddly contrasted with the next key portion of a home, the 'living' room). Other innovations work at the edges of comfortable sleep. Pajamas. The electric blanket. Sleeper cars. Hotels. Means of blocking sound and light; conversely, the alarm clock to counter too-effective efforts to ward off the natural awakeners of sunlight and nature. Science fiction depictions oft include, as a signifier of the alienness of the future world, depictions of sleep occurring in pods, fully enclosed sleek chambers with hydraulic-looking covers in plasticine white or crystal clear, sometimes seeming to present the sleeper on what seems to be some sort of vinyl padding. And, naturally, some fiction revolves around the use of extended periods of sleep to allow the sleeper to pass by mundane years or centuries, whether it by Rip Van Winkle's storied sleep or the centuries of stasis for Khan and his cohorts in the Star Trek universe.

We seem not quite to have gotten to the point of sleeping in pods and giving ourselves centuries of slumber, but the patterns of our lives leave no doubt that all human societies are powerfully shaped by the third of our days, and so of our lives, which we spend in sleep.