I become concerned at the lack of anything from my FA...until I see her turn off the galley light and sit down and pull the blanket around her and close her eyes. She didn't move.

I would be concerned that she was suffering the effects of some sort of medication (legit) and that she had recovered enough by the end of the trip to look and behave in a semi normal fashion.

I would report her only to make sure that she had a medical check up. If she is on some ethical medication ( such as a decongestant) which is affecting her job performance, it needs to be changed before it causes further problems. (If it is some other problem such as laziness, then she will receive notice that it has been observed and needs to be altered.) Just my opinion.
Radonc1