I'm a bit concerned because Nocturne is not a very active cat.  She
spends the days curled on the bed--not moving for 9 hours or more--the
evenings with us, the nights asleep at our feet.

She will not play with us.  If we dangle toys in front of her she has
no interest.  I have seen her playing on rare occasions, but if she
sees me watching her she will walk away from the toy and immediately
ignore it.

The kittens chase and wrestle each other, and Smokey is really into
toys and roughhousing.  Nocturne wants nothing to do with any of it.

She seems athletic when she so chooses--she can easily leap to the top
of our four-foot cat furniture, and chase down the kittens--but most
of the time she just lies there and bites you if you disturb her.

Should I be worried, or is this natural for her?

The vet says she is the ideal weight.

--Fil