It is a little known fact of physics that the mass of a sleeping cat increases exponentially by the following factors: a) how long the cat has been asleep (directly proportional); b) how much you want the cat to move.(inversely proportional). Thus we can use the following equation to calculate the mass of a sleeping cat, where m = the weight of the cat in pounds, t = the amount of time the cat has been sleeping, and i = the appropriateness of the cat's location on a scale of 1 to 10, where 10 is the least appropriate spot.
Thus (m x t)i can be used to determine that a 13 pound cat sleeping for 3 hours on a black wool sweater (10 on the scale) will weigh 390 pounds, but only if you actually try to move the cat. This is because the inertia of a sleeping cat can only be quantified by kinetic energy.
This explains why a cat sleeping on either side of me on top of the blankets is impossible to move. That's my tale, and I'm sitting on it.