False positives for duration percentiles

I’m currently trying to set up alerts based on performance metrics, for example, for the 75p of duration of a certain transaction.
My problem is, that all our users are located in roughly the same time zone, so we do have activity for 12 hours during the day and mostly no activity at night. We still do get some requests at night tough and sometimes they are very slow.
Because we have very little requests, one or two slow requests will trip the alert. This is not technically a false positive, but still something I don’t need to know about.

Is it possible to add additional conditions to alerts? For example, there must be more than X transactions for the given time span, or only enable the alert between a start and end time?