AT A RECENT SLEEP conference,an attendee commented that the function of sleep remains a mystery. The chair of the session argued vehemently against that position—she did not, however, provide a concrete description of exactly why sleep’s function was no longer mystery In humans, a very rare degenerative brain disease called fatal familial insomnia leads to death after several months. Whether the sleep loss itself is fatal or other aspects of the brain damage are to blame is not clear. Sleep deprivation studies in humans have found that sleepiness increases with even small reductions in nightly sleep times. Being sleepy while driving or during other activities that require continuous vigilance is as danger- ous as consuming alcohol prior to those tasks. But existing evidence indicates that “helping” people to increase sleep time with long-term use of sleeping pills pro- duces no clearcut health benefit and may actually shorten life span. (About seven reported hours of sleep a night correlates with longer life spans in humans.) So inexorable is the drive to sleep that achievl arities might also be expected to have similar sleep habits. Yet studies of laboratory, zoo and wild animals have revealed that sleep times are unrelated to the animals’ taxonomic classification: the range of sleep times of different primates extensively overlaps that of rodents, which overlaps that of carnivores, and so on across many orders of mammals. If evolutionary relatedness does not deter- mine sleep time, then what does?
The extraordinary answer is that size is the major determinant: bigger animals simply need less sleep. Elephants, giraffes and large primates (such as humans) require relatively little sleep; rats, cats, voles and other small animals spend most of their time sleeping. The reason is apparently related to the fact that small animals have higher metabolic rates and