Share 'lines in shadow'

Anyone know why I would get lines in the shadows on a 1.5hour rendering for an 8.5 x 11" @ 300dpi very simple render using just the default NXT lighting?  Was expecting to have just a nice shadow fade without the lines.  I think I let it render long enough.  Have to do a lot of these any tips to fix this so I don't have to post process in PS?  Tips much appreciated, thanks.