Sleeping and resting and sometimes just after eating, they will sometimes droop
after eating their meal to digest and rest such as this picture below after eating a whole Silverside.
She is just starting to close up around the fish.
Anemones close up for a lot of reasons.
Usually while they are eating and almost always at night while
I took these pictures of one of my
largest roses all closed up, tentacles sucked in and sleeping
during the early morning hours.
The picture below is from my old picture files from 2000. It is
of an Anemone closing up around a Silverside. They can stay this way for hours so
there is no need to worry.
Sometimes as seen in the pictures above, they won't have any tentacles
showing at all. This is an old old picture from my files .
So just so that you know, they can close around food for hours, or close at night just to sleep.