I recently put together a simple sleep library that does exactly what this poster is looking for. You can find it here:
In this library you can set how long the arduino sleeps for by calling the function sleepDelay(milliseconds of sleep). It works pretty much the same as the delay() function but it sleeps the arduino for that time. It works using the watchdog timer so you can even use it in full power down mode. It includes a few examples to get your started. I hope this helps someone out there! Goodluck!