Dana wrote:
In a way this is irrational--they couldn't all be awake at the same time
Except that, canonically, they nearly always are.
Maybe this is the explanation of the Andromeda phenomenon through which, most episodes, one of the crew seems to have the week off. At first, they used to explain their absence; now it just happens, but perhaps it's just that so and so is having a nice sleep and no one wants to wake them up just to mention they're facing imminent death.