I do leave the computer running the whole time, under the theory that the more it can process in one sitting, the sooner I can be done with it. I'm helped by having another computer for internet and other uses. As for the demo reel, I suppose you could always set up a slide show, but why not produce some animations with Bryce? With a little digging (I haven't given it much thought, so I can't give much information here), you can find the proper aspect ratio, resolution and frame rate to produce an animation that can be output to video.