Sleep is one of the few universal
behaviors. Sleep is seen even in species that would
seem better off without sleep, for example, the Indus
dolphin. Living in muddy waters, over the ages, it
has become blind, presumably because vision is not
useful in their natural habitat. However, despite
the dangers caused by sleeping, sleep has not disappeared.
It never stops swimming; doing so would result in
injury, because of the dangerous currents and the
large amounts of debris carried by the river during
the monsoon season. Studies showed that they slept
a total of seven hours a day, in very brief naps of
four to sixty seconds each. If sleep did not perform
an important function, we might expect that, like
its vision, sleep would have been eliminated in this
species through the process of natural selection.
But why is sleep that important?
Despite centuries of inquiry, scientists
have yet to reach a consensus why we sleep.