I'm using different gravities per area for my game and therefore I need to apply a gravitational force at each fixedUpdate. However, this won't let the rigidbodies sleep. I could manually make them sleep when they are below the sleep threshold (Physics.sleepThreshold), but I don't know how to calculate it for each object. The documentation vaguely says "The mass-normalized energy threshold, below which objects start going to sleep." How is the mass-normalized energy calculated for a given rigidbody?