All animals must rest, but do other animals sleep as human beings do? Observations of animal behavioralone cannot fully answer the question. For example, some fish and snakes never close their eyes whether they areawake or asleep. Elephants may sleep standing up, with their trunks resting in the fork of a tree. 

      The answers may come from experiments in laboratories using a machine called the electroencephalograph(EEG). The machine is connected to animals and measures their brain signals, breathing, heartbeats, and muscleactivities. Using the EEG, scientists have confirmed that all birds and mammals studied in the laboratories dosleep. There is some evidence that reptiles, such as snakes and turtles, do not truly sleep, although they do haveperiods of rest each day, in which they are quiet and unmoving. They have also discovered that some animals, likechimpanzees, cats, and moles, are good sleepers while others, like sheep, goats, and donkeys, are poor sleepers.Interestingly, the good sleepers are nearly all hunters with resting places that are safe from their enemies. Nearlyall the poor sleepers are animals hunted by other animals; they must be watching for enemies, even when they areresting.