A good iOS app for this could be Lucid Reality Check. One just needs to activate the "Autorepeat Alerts" option and also use the "Exclude Sleeping Time" option in reverse, I mean, if I wanted alerts from 4am to 6am I would set the Sleeping Time from 6am to 4am so that I don't get alerts the rest of the day and night. Good thing about this app is it runs in background mode, so no need to leave it open. It has evolved a lot through the years, so the interface might be a bit complex when you start using it. Oh, and it's free.