@Nick and @Crossroads:
It appears, then, that the minimum time is 65 ms (in addition to possibly the number of clock cycles depending on which sleep mode).
But this seems to be very, very long (speaking with respect to typical computer time scales, of course).
For example, suppose I want to accomplish a simple repetitive cycle like this:
--Every 100 ms or so, wake up (via a Timer2 interrupt)
--Do a task that takes about 50 ms
--Go back to sleep
--Repeat
Then, unless I am missing something, the startup time of 65 ms would make the above task impossible (or at least not so efficient in terms of power optimization).
What alternative method could, or is typically, applied for a repetitive, periodic sleep-wakeup-task-then-sleep situation like this? I suppose I'm shooting for a strategy where, after the wakeup-interrupt, the MCU would "startup" in less than 10 ms (as opposed to 65).