Time of Day to define the “between 6am and 10pm” time periods.
Then in your rule
if(vTimeOfDay.state == "DAY" && vPresent.state != ON) // send your alert for daytime
else if(vTimeOfDay.state == "NIGHT" && vPresent.state != ON) // send your alert for night time
See the Design Pattern: Separation of Behaviors for how to centralize your alerting logic so you only have to put the above if else in one place to, for example, use a different alerting based on the time of day. Then in your rule you just vAlert.sendCommand("My alert message!").