Introduction

Abstract

Sleep is an essential part of the human experience. It defines us and links us to other animals who also sleep, although often in ways that are different from ours. For example, some animals, such as dolphins, and some birds, can engage in what is called unihemispheric slow-wave sleep, where they sleep with one half of their brain at a time (Rattenborg, Amlaner, and Lima 2000). Giraffes have a very fragmented sleep, lying down to sleep for a maximum of ten minutes at a time with a total sleep time of 4.6 hours (Tobler and Schwierin 1996, 27). Bats, opossums, and porcupines tend to sleep the most in the animal world with 17 to 20 hours of sleep a day (Zepelin 1989, 85). Whereas humans have about four to five Non Rapid-Eye Movement (NREM)–Rapid-Eye Movement (REM) sleep cycles of around 90 minutes, sleep-cycle lengths differ between animal species, with around 85 minutes for chimpanzees, 72 minutes for elephants, 25 minutes for rabbits, and 12 minutes for rats (Tobler 1995, 38). In 2002, scientists from the University of California, San Diego, found that in a sample of 1.1 million Americans, people who slept seven hours lived the longest (Kripke et al. 2002). Those sleeping for more than eight hours or for less than six hours experienced significantly higher mortality hazards and the risk of early death (2002). Given the central role that sleep has in our lives, it has been the focus of intense scientific research, although the mechanisms and purposes of sleep have remained unclear (Center for Sleep and Consciousness, 2012). So how have we understood sleep?