So you can just disable sleeping altogether on a body by body basis.
This would make sense for your player or things that are controlled, not for physics objects like boxes or whatever. The other option is to adjust the linear/angular damping, so that objects continue moving for longer. This has the affect of delaying the sleep. Finally, you can add a dummy impulse to any physics object (say add an impules of 0.01, 0.01, 0.01 every few seonds) which will wake the object up. This is good if you have an object you want to pickup that may be sleeping, or maybe you don't want to disable sleeping, but you will add this dummy impulse to everything on the screen or within a certain distance from the camera. You can also override _integrate_forces() and do more custom handling of the physics state by hand, but this is usually not necessary.
