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A person stretches while sitting on a bed with daylight coming through windows.Getty Images/iStockphoto

This is the fourth of a nine-part print and online series looking at the science of sleep and the vital role of sleep in maintaining overall health.

Are you a night owl or a morning lark? Or perhaps something in between?

Larks get sleepy soon after supper, but are bright-eyed and bushy-tailed by the time the sun starts to blush on the horizon. Owls are those late-night party animals who thrive in the wee hours, but struggle to get to that 8 a.m. meeting or class the next day.

We all have people in our lives at the ends of the sleep-wake spectrum, but it begs the question: Why do some of us seem built for the morning while others thrive at night?

The answer lies in an individual's chronotype, says Richard Horner, a professor of medicine and physiology at the University of Toronto and author of The Universal Pastime: Sleep & Rest Explained.

Like most other living organisms on the planet, all humans have a circadian rhythm, or "body clock" – a natural, internal system that regulates many of our biological processes over a 24-hour period. A person's chronotype refers to their propensity to wake or sleep during a particular time. While owl types tend toward "eveningness" (going to sleep late at night and sleeping in mornings), larks lean toward "morningness" (getting up extra early and going to sleep early, as well). Others are neutral chronotypes, or somewhere in between.

"Your chronotype is primarily affected by your genes," says Dr. Horner. "You're born with it, like it or not," he says.

"But what's important to know about chronotype in biology is that it can change. It's one of these nature/nurture questions: it's a lot of both."

While many of us function quite well within our chronotype, there can be problems for those who don't.

"If I'm self-employed or I have perfect control over my schedule, it doesn't matter if I go to bed at 2 a.m. and get up at 9 or 10," Dr. Horner says. "What becomes problematic is when someone's biological brain schedule is out of sync with what they are being told to do by their parents, or when they have to get up for a job in the morning."

The ratio of owls, larks and neutral chronotypes in the human population depends on what age group you study, says Valérie Mongrain, director of the Molecular Sleep Physiology Lab at the Université de Montréal, because chronotypes can shift over the course of a person's life. (People often go to bed later during adolescence and earlier in their retirement years.)

The biggest influence on the body clock is the rising and setting of the sun, says Dr. Horner, and since the industrial revolution, chronotypes have also been affected by artificial light sources.

"The eye in the brain doesn't distinguish between natural light and artificial light in this respect," says Dr. Horner. "For example, artificial light at nighttime can influence our body clock and delay it."

"It's a finely operating system, but what people do in our society is they screw it up," says Harvey Moldofsky, president and medical director of the Sleep Disorders Clinic at the Centre for Sleep and Chronobiology in Toronto. "You have people who work shift work, you have people operating on different schedules because we are a 24-hour, industrialized society."

Our habit of taking glowing devices like phones, tablets and laptops to bed with us is likely having a detrimental effect on our sleep cycles, too, Dr. Mongrain says.

"Electronic devices can wake up your brain and delay your circadian clock and this can create bad sleep hygiene," she says. "It's too early to say how much it has an impact, but we will see in the future how it may change the population ratio" of larks, neutrals and owls.

People with extreme sleep schedules can end up with a circadian rhythm sleep disorder, like advanced sleep phase syndrome (for extra-early larks) or delayed sleep phase syndrome (for owls who just can't get to sleep at night). As well, owls who stay up very late on the weekends only to drag themselves out of bed Monday morning for work can suffer from "social jet lag," a problem that's been associated with fatigue, irritability, overeating, obesity and depression.

For parents of adolescents, this phenomenon can be particularly troubling. When they hit puberty, children who previously had no problem getting up in the morning suddenly start staying up later and later, leaving them bleary-eyed come morning.

Colin Shapiro is a professor of psychiatry and ophthalmology at U of T and director of the Youthdale Child and Adolescent Sleep Clinic in Toronto, where he works with young people with a wide range of sleep problems. He says that the owl tendencies of teens can have both a biological and behavioural basis.

To discover what is behind a delayed sleep problem, the clinic runs a dim-light melatonin test, where a person sits in a dark room from 7 p.m. to 3 p.m., providing a saliva sample each hour. "From the saliva, you can measure the melatonin in the system and see the pattern," he says.

People get a rise in melatonin about three or four hours before bedtime, says Dr. Shapiro, a signal to the brain to go to bed in three or four hours. But in some teens, the melatonin rise comes about midnight. "The message to the brain is go to bed at 2 in the morning. Those kids, try as they might, will not succeed in getting to sleep at a reasonable hour. And in most cases, you need a treatment with a hormone like melatonin."

On the other hand, some teens are not naturally sleep-delayed, and they are staying up because they want to, distracted by electronic devices and the promise of fun. "In the same way that some people might prefer to do the night shift because there is less supervision than in the day shift, a lot of adolescents feel that this is time they are escaping the scrutiny of adults, whether it's parents or teachers, and they can do more what they want to do," he says.

One of the biggest problems of this puberty-related delay is that if people continue with this schedule, it can become a reinforced problem into adult life, says Charles Samuels, medical director of the Centre for Sleep and Human Performance in Calgary. "It's not until they get out of university and have to get a job that they end up in our office," he says.

"Unfortunately, we see a lot of these people too late. They are having work-related problems, and winter depression is very tightly linked to delayed sleep phase," Dr. Samuels says. "Maintaining a regular sleep schedule is key, and night owls have a tendency not to do that. They take large amount of stimulants like caffeine to get them through the day."

If you are having difficulty with your chosen chronotype, is it possible to change it? Dr. Samuels says they do just that at the Centre for Sleep.

"You can make a person another chronotype just through environmental influence," he says. Dr. Samuels says it's easier to manipulate the sleep clock of neutral types; owls and larks are harder, "but it can be done" through light therapy and melatonin.

Dr. Modofsky says if you are having difficulty getting to sleep at night, remember the basic things that preserve sleep: proper eating habits, proper exercise and getting outside.

"Most of us work in semi-darkness," he says. "You have to get outside and experience light and exercise, because this helps to safeguard the rhythms of your body."

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