No, actually it seems to enchance their abilities, one recent study discussed how dolphins can click and process received information while sleeping, or can use sonar to search for fish, make decisions about echoes received, while simultaneously whistling, communicating with others. This is a level of complexity that is completely alien to humans.
What definition of sleep is being used here - full cognition (though in a limited domain), motion, conscious response to external stimulii ... doesn't appear to resemble my naive model of 'sleep', in what way is it sleep?
Physiological definition- unihemispheric slow wave sleep, EEG measures each hemisphere's activity, where the sleeping one shows low frequency, high amplitude EEG readings.This is stage 3 sleep. Birds and manatees have it too.
I'm not sure why it would be; as far as I understand it, they don't have a hemisphere asleep all the time.
That said, if I recall correctly, the architecture that supports this capability is more heavily bicameralized than our own. I can easily see that imposing the kind of limit you describe. But I'm not sure we know enough about their connectome to really say one way or another.
I am also not sure if this a limiting or enhancing factor.
But in general, I think, sleep, and 'how much of it' -- was reason (not consequence) of human's cognitive development.
I will admit, I have not found a set of repeatable studies to verify the above assertion. So just a thought.
