At Day’s Close is an excellent book on the history of sleep. It details the evolution of sleep and the impact that technological and social changes (improved lighting, improved law enforcement, and the effects of industrialization) have had on humankind’s sleep cycles.
In effect, natural human sleep begins with “first sleep” in the evening until midnight when people awaken and experience a sense of calmness and creativity which lasts for about an hour before they fall back into their “second sleep” which lasts until dawn. Modern tests have shown this to be true, that our natural rhythms are tied to the transitions of day and night.
Of further interest is the impact that night has on our perspectives, consciousness, creativity, and even our loves. In essence, it strips away the distractions of daylight, societal mores, and enables us to turn inward while experiencing stronger religious, spiritual, sexual, and creative experiences. Many writers and artists, as well as people of other trades and classes, wrote of the special freedom and joy that night brought to them.
The roots of this “paleo” sleep could have come from our need to keep watch throughout a 24-hour period as well as the advantage of hunting at night (human vision is actually quite good, especially peripheral vision).
The value of this bisected sleep was a stronger sense of wakefulness and more energy during the day. With our 24/7 society, being connected with people throughout the day, always on at night, we have lost this unique and special connection with the night as well as losing the benefits of “true sleep.”