It’s a bug (which existed in UDK too btw)… basically a sphere collision shape is never put into sleep.
Internally there should be a sleep threshold that puts physics bodies to sleep when their speed is like 0.00-something but for some reason it doesn’t happen with spheres.
I’m surprised that this bug still exists 
pretty sure it has been reported in the past.
For now I suppose you can add a manual check when the angular velocity is smaller than 0.001 → put physics to sleep… but you should post a bug report in answerhub too.