Multiple requests to sign in to iphone Apple account

How do I stop multiple daily requests on my iphone6 (iOS 11.4.1) asking me to sign in to my Apple account? I sign in, go through the unlocking process, only to get another message a couple hours later asking me to sign in again. I have tried turning the phone off & on but this only delays the problem slightly, giving me perhaps a day without receiving sign-in messages.

JT