At what level of the animal hierarchy do we find true sleep?
Buffalo Grove, Ill.
According to researchers at the UCLA School of Medicine, the presence of sleep has traditionally been judged by a variety of behavioral criteria. Some of these are minimal movement; a typical sleep posture; and reduced responsiveness to external stimulation-a responsiveness, however, that is reversible with relatively intense stimulation (distinguishing sleep from other states such as anesthesia or coma). On this basis, they conclude that sleep is present throughout the animal kingdom.
Only mammals and birds, however, register changes in brain waves and experience the two distinct sleep states-rapid eye movement (REM) and non-rapid eye movement (NREM) -commonly associated with "true" sleep. Reptiles show some brain activity but demonstrate only NREM activity. Other animals with more primitive nervous systems, such as amphibians and invertebrates, don't have brain structures that generate the same electrical patterns of activity, and so the presence of true sleep in these groups is difficult to determine.
Mammals and birds experience sleep quite differently. REM and NREM sleep states are shorter in birds than in mammals: Episodes of NREM sleep average only 21/2 minutes and REM sleep only 9 seconds. Most birds don't lose muscle tone during REM sleep as consistently as mammals do-understandable, since many birds sleep while standing or perching.
Many bird species can sleep unihemispherically-that is, with only one side of their brain. Research at Indiana State University in Terre Haute proved that birds are able to deliberately keep one half of their brain awake as a safeguard against predators. The researchers lined up groups of mallard ducks and videotaped them while they slept. The ducks on the edge of the group were nearly three times as likely as the birds on the inside to keep one eye open-usually the outward-facing eye. When shown an expanding video image simulating a predator attack, the birds reacted within a fraction of a second. Electroencephalograms showed the brain hemisphere regulating the open eye was alert, while the other was experiencing high-quality sleep.
Not as many mammals can sleep unihemispherically; only aquatic mammals such as dolphins, porpoises, and a few species of seal can alternate sleep and wakefulness between the sides of the brain. This allows these mammals to swim to the surface to get air while at rest; research has also suggested that, as with birds, bottlenose dolphins may sleep this way in order to monitor their environment.