Having problem to get your iMac to sleep automatically even though it is set to snooze when idle? If yes, then probably you need to reset a chip that governs a Mac’s power function. It is called System Management Controller (SMC). On most new desktop Macs, what users need to do is shut off the computer, unplug everything such as power, USB and every other cable. Just leave it alone for a minute and then plug it back again.
For a recent Mac laptop, users just need to shut it down and unplug it, remove the battery and then press the power button for at least 5 seconds. Let go of the button, restore the battery and turn the laptop back on.