You can measure whether you're a lark or an owl fairly precisely by taking note of the midpoint of your sleep cycle (what time is it halfway between the moment you fall asleep and the moment you wake up?) and the midpoint of your activity cycle (what time is it halfway between the moment you start being active in the morning and the moment your daily activity phase fades away?). If you don't want to do the measurements, just ask your colleagues or your spouse (or take this quiz).

Chronobiologists measure the activity phase of a person's day by using a wrist actimeter, a device that records the bumps and joggles of the day and uses that to determine the point called "center of activity." The differences between midpoints for larks and owls can be as substantial as three-plus hours.