Height filed doesn't put actors to sleep

Hi Everyone,
My simulation contains the following:
1- One Ground --> PxRigidStatic actor of PxHeightFieldGeometry geometry.
2- Many Rocks --> PxRigidDynamic actors of PxBoxGeometry geometry.

From physx documentation I know that physx put the actors with low kinetic energy to sleep and wakes them up when they are in touch with another actor.
This is very useful strategy to improve the performance (FPS).
Unfortunately this doesn’t work with my simulation because when rocks are dropped on the ground (height field) they are always in contact with with the ground and as such they never go to sleep. Even if I call putToSleep for rocks, physx immediately wake them up as they are continuously in contact with the height field. This results in two side effects:

1- Rocks keep jittering when they on the ground and never go to rest
2- very low FPS.

Is there any solution for this?

Thanks for your help.