do rigidbodies fall asleep at the same time?

Does it just appear to me or do rigidbodies in unity fall asleep simultaneously (meaning, that as long as one rigidbody moves still too fast, all rigid bodies are simulated)?

I have a simulation where multiple rigidbodies are in a container. and when the simulation becomes slower, at some point, all rigidbodies stop at once. It happens when there is not much movement left, but since all of them stop exactly in the same frame, it becomes very obvious to the user. I think if the rigidbodies would fall asleep not simultaneously but when each individually is slow enough to sleep, one would not notice it so much.

Is there something I can do to avoid this? I already set the sleep velocity in the physics settings quite low (0.001) but that didn’t help.

You could try changing some of the settings in the physics settings such as Sleep Velocity, maybe making it a smaller value will make it less noticeable Unity - Manual: Physics

Or you could try setting it manually in a script so when it is below a certain velocity it has a random chance of sleeping:
http://docs.unity3d.com/Documentation/Components/RigidbodySleeping.html