People who are evening types go to bed later and wake up later than morning types. They also tend to move around far less throughout the day, according to an interesting new study of how our innate body clocks may be linked to our physical activity habits.
The study, one of the first to objectively track daily movements of a large sample of early birds and night owls, suggests that knowing our chronotype might be important for our health.
In recent years, a wealth of new science has begun explicating the complex roles of cellular clocks and chronotypes in our health and lifestyles. Thanks to this research, we know that each of us contains a master internal body clock, located in our brains, that tracks and absorbs outside clues, such as ambient light, to determine what time it is and how our bodies should react. This master clock directs the rhythmic release of hormones, such as melatonin, and other chemicals that affect sleep, wakefulness, hunger and many other physiological systems.
Responding in part to these biochemical signals, as well as our genetic inclinations and other factors, we each develop a chronotype, which is our overall biological response to the daily passage of time.
Chronotypes are often categorised into one of three groups: Morning, day or night. Someone with a morning chronotype will naturally wake early; feel most alert and probably hungry in the morning; and be ready for bed before Colbert comes on. Day types tend to wake a bit later and experience peak alertness a few hours deeper into the day. And evening types rise as late as possible and remain vampirically wakeful well past dark.
Our chronotypes are not immutable, though. Research shows that they have a year-long rhythm of their own, with most people harbouring a morning or day chronotype when young, an evening version during adolescence and young adulthood, and a return to a day or morning type by middle age. But some people remain night owls lifelong.
Our shifting chronotypes are known to affect our health, especially if someone is an evening type. In past studies, people identified as evening types were more likely to develop heart disease, obesity, diabetes and other metabolic conditions than people with other chronotypes. They also tended to exercise less and sit far more, which some researchers suspect contributes to their risks for health problems.
But these past studies of chronotype and exercise depended almost exclusively on people’s recollections of how active they had been, which are notoriously unreliable, meaning that any potential links between our bodies’ innate clock signals and our likelihood of moving remained speculative.
Evening types may need to work harder to try to ensure they exercise.
So, for the new study, which was published in June in the Scandinavian Journal of Medicine & Science in Sports, researchers at the University of Oulu in Finland turned to some of their fellow Finns. Years before, more than 12,000 had become part of an ongoing study of the health of almost every child born in Oulu in 1966.
Now, the researchers checked in with almost 6,000 of them still living in the Oulu area and willing to participate in a follow-up study. These men and women, all age 46, visited the university for an in-person exam, which included medical and other tests, and a variety of questionnaires, including one designed to determine their chronotypes.
The researchers also gave each volunteer an activity tracker and asked them to wear it for two weeks, providing objective data about their physical activities. Then, the scientists compared how people moved with how their internal clocks chimed.
And they found that among both men and women, the morning types and many of the day types moved significantly more than the evening types, even when the researchers controlled for people’s health, professions, socioeconomic status and other factors.
Little of this extra activity seemed to be formal exercise, the scientists calculated, based on how much energy the volunteers expended. But it added up. For morning men, the difference amounted to about 30 minutes more of walking each day and for women, about 20 minutes more than among night owls.
The findings underscore that “our chronotypes can have a surprisingly important role in our lives”, said Laura Nauha, a doctoral student at the University of Oulu, who led the new study. They may affect not just when and how willingly we wake, but how frequently we rise from our chairs and move.
This study is observational, though, so it does not show that our chronotypes cause us to move more or less, only that the two issues are related. It also does not explain why evening types tend to be less active, Nauha said.
There may be physiological interactions between people’s body clocks, muscles and other bodily systems that somehow result in evening types being less motivated to get off the couch or chair and stroll.
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But practical considerations probably play a larger part, she said. Evening types may feel most energetic at night, when gyms could be closed and pathways dark. Another obvious factor “could be lack of sleep” and resulting fatigue, she said, since evening types often struggle to sync their body’s timing with the demands of their work schedules – particularly now, during the pandemic, when almost all of our schedules are fractured.
Overall, the study’s findings suggest that late risers may want to monitor how frequently they move, Nauha said. “Evening types may need to work harder to try to ensure they exercise.”
If you are unsure of your chronotype, a version of the questionnaire used in this study is available online here.
By Gretchen Reynolds © The New York Times