If it is necessary to have an animal put to sleep, the owner should be with the animal for the procedure. Once the injection is given, the animal gently slips into a deep sleep, and within a few seconds, the heart stops beating. That is the moment that the animal leaves this life. The vet listens with a stethoscope to the heart to enable him/her to pronounce the death. You can rest assured that once the heart has stopped there is no possibility of the animal waking up again.