One of the most profound insights in all of psychology—and the foundation for much of modern psychotherapy—is this: what we pay attention to becomes our subjective reality. If, when trying to get to sleep, I ruminate on all the tasks on my to-do list, I experience a different reality than, say, if I contemplate a scene in the novel I'm reading. Thinking about the grant proposal I need to revise or the emails I have to return increases my heart rate and gets my brain geared up as if dawn were breaking. Pondering the romance of young Susan and Oliver in Angle of Repose, on the other hand, has the opposite effect.