I assume a FAST is a flow-through/plug-type reactor, and not a batch reactor?
I can only speak to larger, extended aeration AS plants. They go septic very rapidly if air is turned off.
In general, though, it seems to me as if any flow-through AS system needs a constant aeration rate (as opposed to batch systems, which operate differently). This controls the life and eating cycles of the microbes, which must be held fairly constant. Knowing how easily these cycles are disrupted by even small variations of temp, pH, toxins or organic loading, I would have to say "Don't turn the air off".
Remember, amateurs built the ark...professionals built the Titanic. -Steve