Would it be possible to modify the current software to record sleep spindles, or is that outside the range of the hardware?  The study "Spontaneous brain rhythms predict sleep stability in the face of noise" (Thien Thanh Dang-Vu, Scott M. McKinney, Orfeu M. Buxton, Jo M. Solet and Jeffrey M. Ellenbogen. Current Biology, Vol. 20 No. 15, August 10, 2010) appears to find a correlation between sleep spindle frequency and resistance to being awaken by external noise.  The sample size (12) seems a bit small, but Zeo data could probably help corroborate the findings.  Those users with a greater number of sleep spindles should report themselves as heavier sleepers.  If that's the case, anything found to help increase the number of spindles might help light sleepers get a better night's rest.