Long delay to awake iMac from sleep

Hi: Never had this issue before, either with this Mac or any other of our multiple Macs. But now when my Mac is asleep and the screen is dark it literally takes 60 seconds to awaken following keyboard press or mouse movement from sleep and get the screen to change from black back to the login screen. Not the end of the world, but it's irritating to have to wait that long. Any and all ideas most welcome. Thanks!