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Re: Evolution of sleep
Convergence between birds and mammals...great post. Is
there a pill I can take so I can sleep w/ one eye open
to read? Maybe surgery? Don't have time to
evolve...sleepy now...good night.
--- "Richard W. Travsky" <firstname.lastname@example.org> wrote:
> On Thu, 17 Mar 2005, don ohmes wrote:
> > To expand the question:
> > Assuming sleep correlates with sharply reduced
> > alertness/awareness, what is the across-the-board
> > selective advantage of unconsciousness that
> > at least some vertebrate animals from evolving
> > on status relative to mental awareness? Or if
> > are some, why are they rare?
> > Is it a systemic weakness that selection cannot
> > overcome? I can rest w/out sleeping, but sleep I
> > even in the face of certain death.
> Well, I've seen several things speculated as to why
> "shut down" and here
> and there also the disadvantages of doing it. The
> need is strong and some
> species have developed strategies to face it, such
> as hiding somewhere
> in order to sleep safely. I don't know if it
> qualifies as extreme, but
> dolphins and many birds practice "unihemispheric
> slow-wave sleep" or
> sleeping with only half the brain.
> Qwikie search on that -
> Rattenborg, who has studied sleep disorders in
> humans for ten years,
> already knew about the bird-brained capability for
> slow-wave sleep: sleeping with one hemisphere of the
> brain awake while the
> other side experiences the kind of sleep that
> produces large slow brain
> waves. But he wanted to prove why.
> "A study ten years ago found that most birds can
> sleep this way,"
> Rattenborg says. "But we haven't known why, or if
> they can control it."
> Rattenborg decided to test the most intuitive
> explanation for the
> practice: that birds stay half awake to keep alert
> for predators. "If so,
> birds would sleep this way more often in risky
> situations," he explains.
> A risky situation, for a bird, often means being at
> the edge of a group,
> where any predator would get that bird first. So
> Rattenborg took a group
> of mallard ducks -- each in its own clear plastic
> enclosure -- and lined
> them up in a row. He then videotaped the birds over
> the course of four
> nights, switching their positions in the row so that
> each one was, on one
> night, at the end. He also made
> electroechephalographic (EEG) recordings
> of their brain waves.
> The results showed that ducks are not dumb.
> "As expected, a duck at the edge of the group was
> 150 per cent more likely
> to sleep with one eye open," Rattenborg says. "They
> also showed a strong
> preference for looking outwards from the group [with
> the open eye]." At
> the same time, it was 50-50 whether they looked away
> from the group when
> in the centre of the row.
> The EEG recordings confirmed that when one eye was
> open, the corresponding
> hemisphere was in a quiet waking state, while the
> opposite hemisphere was
> experiencing slow wave sleep. And by displaying an
> expanding video image
> simulating a predatory attack, Rattenborg was able
> to prove that the
> wakeful hemisphere was capable of predator
> detection: the ducks initiated
> escape behaviour.
> The ducks would also make sure to alternate which of
> their halves was
> getting shut-eye -- and shutdown brain activity --
> by rotating their
> bodies to switch the side that was facing outwards.
> "The birds would switch back and forth between
> sleeping with the left and
> right hemispheres," Rattenborg says. "You'd see them
> stand up every so
> often and turn around, to keep the other eye open,
> and give the other half
> of the brain some sleep."
> But how do they do this?
> "We don't know," Rattenborg admits. "But we do know
> that when birds are
> awake, they can look up with one eye and down with
> the other, operating
> the two halves of the brain independently. They
> appear to have more
> separate brain halves than most mammals."