Frequent Sign-In Required

Greetings. iMac late 2013 now running Catalina (but this was also true when running High Sierra). Many times, although not always, if I go away from my iMac for say an hour, when I return it requires me to sign in again and then goes through the whole start up procedure from the beginning. Very time consuming. Once each morning is ok, but this happens repeatedly during the day.....help!