SPS Polyp Extension

[Baldy]

I am quite new to SPS. the 2 frags i have now are surviving which is more than i can say for the other 3 SPS frags ive tried. My question is regarding polyp extension. i have noticed the last 2 days or so that the birdsnest and digi that i have arent quite extended as much as they were the first 2 weeks i had them. This could very well be my imagination and me staring at my tank to much and thinking i see things that arent there.

do SPS act like LPS and softies? do they retract sometimes like at night, or after a change in flow? ive noticed that when bumped by snails or something they retract as expected, but i was just wondering what others experience. are yours extended fully most of the time?