Study links weight to when the school bell rings | Science News for Students

Study links weight to when the school bell rings

Teens and preteens who start school earlier weigh a bit more
Sep 17, 2018 — 6:45 am EST
the back of a schoolbus driving on a street before sunrise

Many students start school earlier than experts recommend. A new study shows that earlier start times are linked with being slightly heavier.


Teens and preteens who start the school day really early tend to weigh slightly more than those who start later. That’s the finding of a new study of nearly 30,000 Canadians between the ages of 10 and 18.

These data are the latest evidence that starting school too early can harm health.

“We know from earlier studies that when school starts too early, kids can’t get the sleep they need,” says Geneviève Gariépy. She works at McGill University in Montreal, Canada.

Getting too little sleep can put kids at risk for a number of problems. Sleepy teens are more likely to be overweight, to have trouble concentrating and to struggle in school. They also are at higher risk of feeling depressed, using alcohol or drugs and getting in car accidents.  

As an epidemiologist, Gariépy is interested in patterns of disease. She studies teen obesity, or extreme overweight. In this study, she wanted to know how earlier school start times might be linked to weight.

“Overweight and obesity in adolescents is a big problem in North America,” she says. The number of overweight kids has grown over the past 30 years. About one in three U.S. and Canadian teens are now overweight or obese. Kids who don't sleep enough may be at higher risk. So Gariépy decided to home in on the impact of school start times.

Weighty findings

Her team collected start times for 362 Canadian schools. Then they asked students at those schools to give their height and weight. In all, they collected data from nearly 30,000 10- to 18-year-olds.

Among 6th- to 10th-graders, those who started school earlier tended to be slightly heavier for their height. Every 10-minute delay in school start time was linked, on average, to a slightly lower weight among students who were the same sex, age and height.

What does that mean in real numbers? Think, for example, about two 14-year-old girls. Both are 161.2 centimeters (about 5 feet, 3 inches) tall. The one who starts school at 9:30 a.m. could expect to weigh 1.7 kilograms (3.7 pounds) less than girl who starts school at 8 a.m. The same pattern held for boys.

Gariépy’s team published its results in the July Journal of Adolescent Health.

She also notes that it’s impossible to know whether earlier school start times actually caused the difference in weight. It might simply be a correlation. That is, children who started earlier in the morning might have been heavier for other reasons.

Still, Gariépy says “Our study suggests that if all the schools shifted and let students start half an hour later, it could help everyone be a little leaner.” However, she adds, the difference is slight. And healthy eating and getting enough exercise will play a bigger role in maintaining a healthy weight.

Because the study compared kids who already started school at different times of day, it’s not clear that changing school start time would help heavier kids lose weight. To know for sure, researchers would want to compare the same kids before and after changing to a later school start time.

Nonetheless, the study is a good start, says Cora Collette Bruener. She’s a pediatrician at Seattle Children’s Hospital in Washington. The findings add to a growing pile of evidence that later school start times might be better for teen health, she says.

The sleep-weight connection

What might link earlier school start times with weight? Gariépy suspects it’s sleep, though she can’t say for sure. After all, her team didn’t ask students in this study about their sleep habits.

Bruener agrees that early school start times make it hard for adolescents to get the recommended eight to 10 hours of sleep each night. And as they get older, their bodies naturally want to stay awake later and to wake up later. That’s because of changes due to puberty.

“Hormones in the brain that assist in falling asleep aren’t secreted until later at night during puberty,” says Bruener. Those changes result in a natural sleep period between 11 p.m. and 8 a.m.

For this reason, the American Academy of Pediatrics recommends that middle schools and high schools start no earlier than 8:30 a.m. Yet many U.S. and Canadian schools do.

Kids have no control over when their schools start. But Bruener says they can take some steps to protect their sleep.

To get a good night’s shuteye, she recommends putting away electronics 30 minutes to one hour before bedtime. (The light from screens and electronics can make the brain resist sleepiness.) Also, she says, these devices should be kept in a different room than where you sleep. That’s because text messages and apps can ding and buzz. These sounds can keep you awake, even when you aren’t looking at the screen.

Power Words

(more about Power Words)

adolescent     Someone in that transitional stage of physical and psychological development that begins at the onset of puberty, typically between the ages of 11 and 13, and ends with adulthood.

app     Short for application, or a computer program designed for a specific task.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

correlation     A mutual relationship or connection between two variables. When there is a positive correlation, an increase in one variable is associated with an increase in the other. (For instance, scientists might correlate an increase in time spent watching TV with an increase in rates of obesity.) Where there is an inverse correlation, an increase in one value is associated with a drop in the other. (Scientists might correlate an increase in TV watching with a decrease in time spent exercising each week.) A correlation between two variables does not necessarily mean one is causing the other.

epidemiologist     Like health detectives, these researchers figure out what causes a particular illness and how to limit its spread.

high school     A designation for grades nine through 12 in the U.S. system of compulsory public education. High-school graduates may apply to colleges for further, advanced education.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.

link     A connection between two people or things.

middle school     A designation for grades six through eight in the U.S. educational system. It comes immediately prior to high school. Some school systems break their age groups slightly different, including sixth grade as part of elementary school and then referring to grades seven and eight as “junior” high school.

obesity     (adj. obese) Extreme overweight. Obesity is associated with a wide range of health problems, including type 2 diabetes and high blood pressure.

overweight     A medical condition where the body has accumulated too much body fat. People are not considered overweight if they weigh more than is normal for their age and height, but that extra weight comes from bone or muscle.

pediatrician     A field of medicine that has to do with children and especially child health. A doctor who works in this field is known as a pediatrician.

puberty     A developmental period in humans and other primates when the body undergoes hormonal changes that will result in the maturation of reproductive organs.

sex     An animal’s biological status, typically male or female.

shuteye     Slang for sleep


Journal: G. Gariépy et al. School start time and the healthy weight of adolescents. Journal of Adolescent Health. Vol. 63, July 2018, p. 69. doi: 10.1016/j.jadohealth.2018.01.009.