The Cloud Ark passed through a complete day/night cycle every ninety-three minutes. Time was arbitrary in space, so the ISS had long ago settled on Greenwich time, also known as UTC, as a reasonable compromise between Houston and Baikonur. The Cloud Ark had inherited that system, and Day 700 began at midnight at the Royal Observatory in Greenwich, or A+1.335.0 in ark time. About one-third of the population woke up to begin a sixteen-hour shift. Others would wake up at A+1.335.8, or "dot 8," and at "dot 16." The system ensured that about two-thirds of the population was awake at any given time. Awake people needed more oxygen and generated more heat than sleeping people, so it put less of a strain on life support systems, and enabled the Cloud Ark to support more people, if the waking and sleeping cycles were staggered. A reason for the popularity of triads was that each of the three component arklets could operate on a different shift, observing its own artificially imposed period of darkness and quiet. In a heptad, the same basic scheme could be used, with two of the arklets asleep at any given time and the one in the middle of the hexagonal frame being "always on."