It's the way of kittens that ten minutes of craziness is followed by about three hours sleep.
You can usually find some active bits by moving the Play button backwards though.
They are loving the Tardis at the moment, although they were at the vet recently and had their claws trimmed, so it's not so easy for them to climb at the moment. The last batch of kittens (Ripley's kittens) used to run up the side of the Tardis and I expect these ones will when their claws grow back a bit.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal