The study authors speculate that "segmented sleep" was an adaptation to longer nights in the northern hemisphere. That is the pattern where you fall asleep until the middle of night (1-4ish) then wake up for a while, then fall back asleep again--as documented in At Day's Close: Night in Times Past by A. Roger Ekirch. I've been very interested in this, as I've had that pattern for a long time, and trying to figure out if it is a natural adaptation or not. Good time to get reading done!