Well, ocean currents don't necessarily take a break at night (though it's true that in some places things can get calmer), so I think most SPS corals can handle lots of night time flow no problem. However, if you turn the current down you'll notice quite a bit more night time polyp extension. I'm sure long tentacled LPS will appreciate the reduced flow, though most of mine retract almost all the way at night regardless of what the flow is doing.
I turn the flow in my tank down at night mostly for a single fish that seems to struggle with too strong of a flow at night.
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