Why the interest in rest and sleep? - All living things possess a sense of time. Like all animals - as well as plants, fungi and bacteria - we humans are hard-wired for a daily rhythm of rest and activity. The machinery that generates this daily rhythm resides in each and every one of our cells and it powerfully orchestrates our behavior. Setting aside time for rest and sleep optimally fits all living things and their bodily processes to the conditions of life. In fact, rest and sleep are deeply rooted in ancient biology, so much so that these characteristics have been conserved over the deep time that spans the three and a half billion years of life on earth. The dawn of the 24-hour industrial and technological society, however, occurred less than one hundred and fifty years ago. This time span constitutes only three human generations, and is but a ‘blink of an eye’ on the grand scale of natural history that spans billions of years. As a result of the technological age, humans are now the only living things that purposely ignore, disrupt and coerce the natural cycles of our cellular machinery to conform to artificially-imposed and often-irregular daily schedules. Given that sleep has major influences on body and mental health, the implications of understanding the states of rest and sleep, and why they exist, are large.