Sure, you can use multiple SLEEP periods and keep track of them, just remember that the SLEEP period isn't very accurate and may (will) change with temperature and even from device to device but in your application I don't think it matters if it differes an hour or two in the end.

There are of course other ways of doing it but, if accuracy isn't an issue, none that is as convenient as SLEEP - not that I can think of anyway.

/Henrik.