Sleep when needed
Chronotype determines more than when you sleep and wake. It orchestrates predictable peaks and troughs of energy over the course of the 24-hour day. The so-called "window of circadian low" -- the hours when the body is least adapted for wakefulness -- typically occurs between 2 a.m. and 6 a.m. There's another, smaller dip 12 hours later, in the midafternoon.
There are also two high points, when thinking is sharp and reaction times quick. One occurs within an hour or two after waking, and the other after the daytime dip. This cycle is shifted earlier in a morning person and later in an evening or night person.
At the Denmark offices of the pharmaceutical company AbbVie, employees design work schedules that take advantage of their biological strengths.
Sticking to traditional hours can be counterproductive, leading to "presenteeism" -- employees showing up and being only minimally functional. "Companies are wasting the potential of their people," Dr. Volk said. "You have someone sitting there from 7 til 9 a.m. sipping coffee, being completely unproductive, and then you send them home at 4 when they actually start getting productive."