This is myopic. In many cases it takes time, and sometimes considerable programming effort, to enter and exit low power modes. So you don't do it willy-nilly; you do it when you believe the system has quiesced. That means, on a purely interrupt driven system that is not yet ready to sleep, the code may very well be spinning in an empty infinite loop somewhere.

There's no need to inform the "user" because there's nothing wrong with the system. Its simply waiting until the benefit of sleeping outweighs the cost of getting there.