The Wall Street Journal reports on the new science behind the disruption wrought by jet lag, unusual hours and bright tablet screens.

Fast-forward to the modern era, when work schedules almost ensure that the day begins for most of us with an alarm clock interrupting our sleep. Add to this substantial and increasing light exposure at night, rapid travel across time zones, rotating shift work, and delayed and irregular meal times, and we have the ingredients for circadian disruption: a state in which the body’s many internal clocks fall out of sync with one another and with the external daylight cycle, and—in some cases—stop functioning entirely.