According to A. Roger Ekirch
Dr. Thomas Wehr and his colleagues at NIMH found that human subjects, deprived at night of artificial light over a span of several weeks, eventually exhibited a pattern of broken slumber […] practically identical to that of pre-industrial households. Without artificial light for up to fourteen hours each night, Wehr’s subjects first lay awake in bed for two hours, slept for four, awakened again for two to three hours of quiet rest and reflection, then fell back asleep for four more hours before finally awakening for good. Significantly, the intervening period of “non-anxious wakefulness” possessed “an endocrinology all its own,” with visibly heightened levels of prolactin, a pituitary hormone best known for permitting chickens to brood contentedly atop eggs for long stretches of time. In fact, Wehr has likened this period of wakefulness to something approaching an altered state of consciousness not unlike meditation.