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Re: Evolution of sleep



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 prevents
> at least some vertebrate animals from evolving always
> on status relative to mental awareness? Or if there
> are some, why are they rare? 
> 
> Is it a systemic weakness that selection cannot
> overcome? I can rest w/out sleeping, but sleep I will,
> 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 -

http://www.exn.ca/Templates/Story.cfm?ID=1999020351
...
Rattenborg, who has studied sleep disorders in humans for ten years,
already knew about the bird-brained capability for unihemispheric
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." 
...