Uplay SH5 minimizes to desktop after 5 minutes
 

I didn't used to have this problem so I'm not sure what's happened. If I don't use my mouse etc in game for about 5 minutes, the game minimizes (not a crash) to desktop, as if you've pressed ctrl + esc. It's not a problem continuing the game but there are times I just want the game to run in 1X time while I do something else, such as when destroyers are around.

Any ideas?

Thanks.