Any ideas or tips? Is there a way in the light settings or do I have to use several light sources? I don't know if several light sources will help me though - I'm trying to make the shadows less perfect/sharp. thanks!

Try Zenith, try rendering at premium with soft shadows, try adding some ambient lights (omni lights with low intensity), normally you need a few fill/ambient lights in a scene which knocks up render time.

If you use the premium render functions, use the soft shadows at or above 36 rays per pixel. Below 36 the render produces very noticable artifacts.

Here's two tutorials I wrote a while back for Bryce4:
Simulating Occlusions in Bryce
Make a Dome Light

Their pretty old now that Bryce5 is around, but they still work, and may even save some rendering time vs using the True Ambience settings.

At least they may give you some ideas to try.
Have fun.

U wrote that tut? Great work and a very different take on the lighting!

A now famous thread for illumination techniques in Bryce can be found here.

great dome tut pada