Two pilots flying on long flights

Suppose you're PIC with a pilot friend on a long flight. You decide
that to save time, one of you can nap while the other continues the
flight. The aircraft requires only one pilot. Is this legal? In
particular, can the pilot in command sleep during the flight? Or does
the active pilot become the pilot in command while the other pilot
sleeps?

Does anyone actually do this on long flights? I suppose the need for
a toilet stop might make it a rare situation. In cars, though, I know
that it's perfectly common for several drivers to share driving
duties, with one napping while the other drives.

Along the same lines ... what is the major limiting factor on range
and flight duration for small GA planes? Is it fuel? The
availability of suitable weather along one's route? The need to land
and find a restroom or sleep?

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