Our bodies have an inborn reflex that organizes all our bodily stress. It’s called the startle reflex. I remember reading a study several decades ago where scientists examined startle reflex among human subjects. They would wire volunteers up, strike a large bell, and record their physiological changes: muscles clench, sudden deep breath, and we're were startled. As the testing continued, the changes that occurred as the bell rang again and again diminished over time. The subjects became habituated. They came to expect the bell and were no longer startled. They didn't even notice it!
Another group was studied in the same way. This group consisted on Buddhist monks, who had spent years learning and practicing meditation. When the bell was rung the first time, they exhibited the startle response, as had the other group. The difference was that the monks response was the same on the tenth time as it was on the first. They did not become habituated. They stayed alert.
Throughout this Advent, may we, through prayer and meditation, stay alert and be ready for whenever our Lord comes again.