I found with the Angelcare monitor that it always went off if she was up against a side or in a corner. A couple of times it went off when she was in the middle and I assume it was because she was in a deep sleep as she moved with the beeping and it stopped. I guess without the monitor I just wouldn't have known she was in a deep sleep, but it's nice to know it helped if she actually did need a little kickstart, so to speak. I was happy to keep checking when it went off and of course, my own personal need for reassurance just gradually went away as she got older until I was comfortable with not using it anymore.