Not all experts acknowledge such fine-grained distinctions, instead grouping people into morning, evening and “intermediate” types. Regardless, while true “larks” and “owls” tend to dominate the conversation, they make up a small percentage of the population.
To determine your chronotype, imagine that you have two weeks of vacation to spend as you like, with no evening or morning commitments and no pets or children to wake you. Chronotypes reflect habits as well as biology, so you would also need to eliminate caffeine and avoid artificial light at night, which pushes a person’s chronotype later. At what time would you tend to fall asleep and wake up? Don’t be surprised if you’re unsure. After years spent accommodating work, family and social commitments at the expense of sleep, “a lot of people don’t know what rhythm they have,” Ms. Kring said.
Chronotypes shift in a predictable way over the course of a lifetime. Between the ages of 12 and 21, everyone’s natural sleep schedule gets about 2 1/2 hours later — which is why adolescents have so much trouble waking up for school. After that, chronotype creeps in the other direction, which is why older people typically find themselves waking earlier than they used to.
But 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. A nine-hour training program helps them identify when they are ripe for creative or challenging projects, typically mornings for early risers and afternoons for late risers. Lower-energy periods are meant for more mundane tasks, like handling emails or doing administrative chores. Workers save commuting time by avoiding rush hour traffic, and can better mesh their personal and professional lives — for example, by getting their children from school in the afternoon, then working from home in the evening after the kids are in bed.
Employee satisfaction with work-life balance has risen from 39 percent 10 years ago, when the program launched, to nearly 100 percent today, according to company surveys. Last year the Denmark division of Great Place to Work, a global organization that ranks companies based on employee satisfaction, named AbbVie the top middle-size company in the country. “The flexibility actually empowers people to deliver the best possible results,” said Christina Jeppesen, the company’s general manager.