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People who are productive later in the day tend to burn less fat than early risers. d3sign/Getty Images
  • Early risers burn more fat, even while resting, than people whose chronotype is to be busy in the evening.
  • Differences in how fat and carbohydrates are burned can have consequences for disease risk.
  • Chronotypes can’t be completely changed, but scheduling around your chronotype could have health benefits.

It’s been said, “The early bird catches the worm.” But it’s mostly early birds who’ve been saying it.

Night owls think, “Forget the worms, I’m going back to bed.”

If one of these sentiments resonates with you, it probably indicates your chronotype, or when you’re most productive during the day.

If you’re an early chronotype — a so-called “early bird” — then you’re more productive early in the day. If you’re a later chronotype — a “night owl” — you’re more productive in the afternoon.

Later chronotypes tend to have a lower level of insulin sensitivity, which can lead to a greater risk of type 2 diabetes and cardiovascular disease.

Now, new research published in the journal Experimental Physiology attempts to explain why.

In this study, researchers used a questionnaire to divide participants into two groups based on their chronotype, either early or late.

The participants maintained a low-fat diet for the study, fasting overnight and undergoing testing in the morning.

Testing consisted of taking blood samples as well as capturing respiratory gases after exercising.

The results suggested that participants with early chronotypes tended to burn more fat — both during exercise and while at rest — than their later chronotype counterparts.

Those with later chronotypes were more likely to get their fuel by burning carbohydrates.

This is despite the fact that all participants, regardless of chronotype, took in about 55% of their daily calories from carbohydrates and 30% from fats (with less than 10% saturated fat).

These results were independent of each individual’s level of physical fitness or how much energy they used while exercising.

So even if you’re very physically fit or if you burn a lot of energy while you exercise, your chronotype is still likely to have an effect on what type of fuel your body favors.

There is an important distinction to be made here between circadian rhythms and chronotypes.

To a certain degree, circadian rhythms can be altered, but that’s not so much the case for chronotypes.

“What time you’re actually awake is your circadian rhythm. Our circadian clock dictates a wake-up time and a sleep time,” said Dr. Aatif Husain, a professor of neurology and the division chief of Epilepsy, Sleep, and Neurophysiology at Duke University School of Medicine in North Carolina.

“The ideal time for you to be productive is your chronotype,” Husain told Healthline. “If you’re a night owl but you have a job that starts at six in the morning, you may force your circadian rhythm to behave differently than your chronotype, and that’s where you’re going to start getting into problems.”

Where do our chronotypes come from?

“Chronotypes are determined by our genetics and, to some degree, our environment,” Dr. Stephanie M. Stahl, a sleep medicine physician at Indiana University Health, told Healthline. “Chronotypes are mostly static, but there is a shift toward a later chronotype in teens and an earlier chronotype in older adults.”

Our chronotypes are generally not as extreme as you may think.

“Certainly there are people who can do things at 10 p.m., but when we’re talking about the late chronotype we’re talking about productivity between noon and 4 p.m. On the other hand, the early birds are going to want to get up early and get the most important stuff done and out of the way as soon as possible,” explained Husain.

“We often refer to ourselves or others as ‘early birds’ or ‘night owls,’ but what gets lost is that the vast majority of people are somewhere in between,” he added.

While burning either fat or carbohydrates may explain some of the difference in health trends between early and late chronotypes, it’s not the whole story.

“The issue is that night owls have to live in an early bird’s world where work starts at 8 a.m. and ends at 5 p.m. Your body is not living in the same time that your chronotype is living in, and you chronically deprive yourself of sleep,” said Husain.

So what’s a night owl to do?

“We can make changes in our routines as well as light exposure that may help us adjust to a schedule,” said Stahl.

“For most people, exercising within one hour of bedtime can negatively impact sleep. Ensuring that you get seven or more hours of sleep, optimizing your sleep quality, and treating sleep disorders such as obstructive sleep apnea may help reduce the risk of heart disease and diabetes,” she added.

“Making yourself wake up early and exercising at 6 a.m. every day won’t make you an early bird. Understanding and trying to live healthy within your own biology is going to be the better way to do it,” said Husain.

“If you’ve done all the things that are reasonable in terms of trying to optimize your sleep health and your wakefulness health and you’re still not feeling great, seeking medical help is the way to do it,” he added.