Peter Nilges has done some scientific testing in his dissertation (at the German College of Physical Education, Cologne) on the rolling resistance of mountain bike tyres under varying conditions. The results were a surprise.
While on tarmac thinner, higher pressure tyres have a lower rolling resistance the opposite is true for off-road riding.
Another reason to switch to tubeless, which not only helps prevent punctures, but can be run at a lower pressure (and depending on the system you use can save weight, or more importantly, rotational mass).