29 02 12 - 19:20
Lead author Michael A. Grandner of the Center for Sleep and Circadian Neurobiology at Perelman School of Medicine at the University of Pennsylvania said the findings suggested in general those in the South were most likely to report sleep disturbance and daytime fatigue. Those in the West were least likely to experience fatigue. - UPI
I think it's related to heat, humidity and the fact that all known air conditioners shut off two hours before dew point and thus render the home inhospitable at the exact time when people go into the deepest sleep. I've never been in a home that did not do this. I'm not sure why it happens, except that for whatever reason the measurement of house temperature is cooler (possibly the wall material itself cooling) than the air in the house.