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Body and mind

People born prematurely are more often larks

New Finnish research shows that adults who were born preterm at very low birthweight have an earlier circadian rhythm.

If you were born before the 37th week of pregnancy and weighed less than 1,500 grams at birth, the odds are that you are a morning person.

This is the conclusion of a new study in which researchers investigated how birthweight and early versus full-term birth affects a person’s circadian rhythm later in life.

This discovery may be important for several reasons.

Premature birth is linked to higher blood pressure later in life, poorer blood glucose control, less exercise and a greater tendency of having depression, whereas being a lark is associated with a healthier life, so these two tendencies might to be able to offset each other to some extent.

“Our study showed that adults born prematurely with very low birthweight display an earlier circadian rhythm but have the same duration and quality of sleep as people who were born full term and with normal birthweight. The earlier circadian rhythm could potentially provide some protection against the negative outcomes of being born prematurely at very low birthweight,” says sleep researcher Johan Björkqvist, Children’s Hospital and Pediatric Research Center, University of Helsinki, Finland.

The research has been published in Chronobiology International.

Many factors can influence circadian rhythm

People’s variability in their circadian rhythm is also known as their chronotype, which can range from extreme morningness “larks” to extreme eveningness “owls”.

The chronotype is a person’s temporal (time) preference for activity and sleep during the 24-hour day, and although this can change throughout life, and genetics plays a major role, what happens during pregnancy could explain a part of what shapes your personal chronotype.

Experiments on mice have indicated that maternal stress during pregnancy, fetal malnutrition, alcohol consumption, lack of oxygen and continuous exposure to light after birth can affect the offspring’s chronotype.

Directly comparable studies have not been performed among humans, but researchers have found that children and teenagers born prematurely might have an earlier chronotype.

“Various research groups have used different methods to study quite different populations, so to bring more clarity to the issue we decided to carry out the most comprehensive study to date” explains Johan Björkqvist.

Measured circadian rhythm over 2 weeks

The study measured the circadian rhythm of 53 people who were born prematurely with very low birthweight matched with same-sex full-term siblings and 17 unmatched participants.

The participants, who averaged 30 years old, were equipped with an actigraph unit for 2 weeks, tracking their sleep patterns, including when they fall asleep and wake again.

Johan Björkqvist says that the sleep midpoint is a good indicator for the circadian rhythm.

The results showed that people who were born prematurely with very low birthweight had a sleep midpoint that averaged 30 minutes earlier than their siblings. After the researchers adjusted the results for sex, age and various factors related to the quality of sleep, that number rose to 40 minutes.

The result was especially evident among those who had been below the normal weight-for-age curve as newborns and among women.

“Based on the results, we conclude that being born prematurely with very low birthweight seems to be associated with chronotype,” says Johan Björkqvist.

A potentially lifelong trait

According to Johan Björkqvist, previous studies have not obtained such conclusive results as the current research.

This may result from the fact that researchers have examined sleep patterns among children and adolescents, and these can vary greatly over time.

As many parents probably know, young children often get up far too early, whereas getting teenagers out of bed can be a struggle.

Nevertheless, previous studies on children and teenagers have also indicated that premature versus full-term birth and birthweight might influence chronotype.

“Our participants are the oldest to have been investigated in this way. Our findings show that the previously observed trait persists long term, even after children leave the parental home and live independent lives. We also assume that the trait does not result from factors in upbringing, since we controlled for this variation by matching the preterm subject to their own sibling,” explains Johan Björkqvist.

More data to analyse

The big question is what biological mechanisms in the womb influence the circadian rhythm in later life.

The new study did not answer this now, but the answer may emerge later.

In the current research, Johan Björkqvist and colleagues not only examined the participants’ circadian rhythm but also had them complete questionnaires; examined them by using dual-energy X-ray absorptiometry (DEXA) and magnetic resonance imaging (MRI); and performed genetic analysis and biopsies.

Hopefully, all these tests and studies will provide part of the answer, and the researchers are working to discover this in the near future as they process the large quantity of data.

“It will be interesting to see what else we can find out,” says Johan Björkqvist.

Chronotype in very low birth weight adults – a sibling study” has been published in Chronobiology International. In 2018, the Novo Nordisk Foundation awarded a grant to co-author Eero Kajantie for the project Small Preterm Infants and Their Siblings – a Natural Experiment to Understand the Life-long Legacy of Early Life Environment.

Johan Björkqvist
Post. doc.
The Children’s Hospital is among the largest units in the Nordic countries focused on teaching and studying medical conditions in children and adolescents. It is responsible for basic education and specialist training at the Faculty of Medicine in pediatrics, pediatric surgery and neurology, pediatric and adolescent psychiatry, as well as pediatric infectious diseases. The hospital conducts active research operations and is responsible for postgraduate research education in the field of children and adolescents. A doctoral programme also operates within the purview of the Children’s Hospital. The Children’s Hospital operates in facilities provided by the Hospital District of Helsinki and Uusimaa, primarily at the Hospital for Children and Adolescents and at the Children’s Castle. In pediatric psychiatry, the unit also has facilities elsewhere in Helsinki. The hospital conducts close and seamless cooperation with the hospital district. In addition to treating people living within the actual confines of the district, the hospital district carries the nationwide responsibility for treating pediatric patients that require demanding cardiac surgery and other surgery procedures, as well as pediatric oncology and organ transplantation patients.