Amber alerts are bad. We get them now and them in Ontario at 3am, sent to the entire (rather large) province. This is idiotic. The probability that someone would be awake and in a position to observe an abducted child at 3am is negligible. The probability that someone woken at 3am will not be able to get back to sleep, drive to work in the morning when drowsy, and consequently get into an accident that kills someone, is not negligible.
Such alerts should be confined to warnings that people really do need to pay immediate attention to. Like a train accident that has released toxic gas, requiring immediate evacuation, even if it's 3am. As it is, an alert at 3am will be ignored, since the system has been abused by the idiots running it.