One question which I don't think has been adequately addressed is the
question of whether REM sleep exists in dolphins. Since REM sleep results
in muscle atonia, and dolphins are continuously swimming, perhaps it is
not surprising that they show no overt evidence of REM sleep. However, my
own observations of the pigeon lead me to believe that the existence of
REM sleep may be easy to miss. In pigeons, REM sleep last for as little
as 3 seconds at a time. I would hypothesize that perhaps this is an
adaptation for remaining on an unsteady perch while sleeping.
You propose that sleep represents some critical aspect of our physiology,
and is not simply an evolutionary adaptation. Keeping in mind that by
definition it must be some kind of adaptation, there are several lines of
evidence which indicate that sleep is not, at least by definition, a
critical process. The amount of sleep required by individuals is extremely
variable - some individuals are capable of functioning perfectly normally
on one hour or less of sleep per day (I speak of humans), perhaps
indicating a lack of any strong selective pressure. Pigeons can be
completely sleep deprived for over a month simply by keeping them in
conditions of continuous light, and yet they show no overt ill effects,
and show no sleep "rebound" after being returned to darkness.
In fact, some researchers would even claim that dolphins aren't really
sleeping at all while showing the unihemispheric slow wave activity
(Personally, I do think they are sleeping - this is for discussion's
sake). In their minds, if the dolphins are swimming and showing such
coordinated movements in a social group than by definition they are not
asleep. There is also precedent for high levels of slow wave activity
during waking, as this is quite common in birds.
I am not saying that sleep is not a critical physiological process in some
cases, only that we should be cautious before trying to generalize about
the properties of sleep across different species.
Eric Mintz | "Run! Seek Shelter! The 7-year dung
Department of Biology | frogs are migrating!"
University of California, Santa Cruz | -- Robotman (actually, his tour guide)