you might want to consider temperature as a possible culprit. a lower temperature is a signal to the body to curl up and sleep, and a consistently cool (relative) temperature keeps you in that state. when we wake up, our bodies ramp up our metabolism/temperature to rouse us out of slumber. i know i'm ready for sleep when i impulsively reach for the sheets. with two people, a dog, and only a cracked window, the temperature might be rising above your sleep signal during the night (i wake up often in the middle of the night and can literally feel the heat rising as i gain consciousness).