Is it better for our bodies to work out at certain times of the day? Scientists have known for some time that every tissue in our bodies contains a kind of biological clock that goes off in response to messages related to our daily exposure to light, food and sleep.

However, whether and how exercise timing might influence metabolic（新陈代谢的）health has been less clear, and the results of past experiments have not always agreed. A much-discussed 2019 study found that men with Type 2 diabetes（糖尿病）who completed a few minutes of high-intensity interval（间隔）sessions in the afternoon improved their blood-sugar control after two weeks. Patrick Schrauwen, a professor of nutrition and movement sciences read that 2019 study with interest. He had been studying exercise in people with Type 2 diabetes, but had not considered the possible role of timing. Now, seeing the varying impacts of the intense workouts, he wondered if the timing of workouts might similarly affect people’s metabolisms.

Incidentally, he and his colleagues had a ready-made source of data in their own prior experiment. Several years earlier, they had asked adult men at high risk for Type 2 diabetes to ride stationary bicycles at the lab three times a week for 12 weeks, while the researchers tracked their metabolic health. They also had noted when the riders showed up for their workouts. The researchers pulled data for the 12 men who consistently had worked out between 8 and 10 a.m. and compared them with another 20 who always exercised between 3 and 6 p.m. They found that the benefits of afternoon workouts far outweighed those of morning exercise.

He says, “This study does suggest that afternoon exercise may be more beneficial for people with disturbed metabolisms than the same exercise done earlier. The particular and most effective exercise for each of us will line up with our daily routines and exercise tendencies because exercise is good for us at any time of day — but only if we choose to keep doing it.”

Recent studies found that smiling at London bus drivers increases happiness. However, on the Number 24 bus to Hampstead Heath, Londoners are sceptical. “Bus drivers,” says. Liz Hands. a passenger. “are generally annoying me.”

It might seem improbable that a report on London’s buses could change behaviour. But it has happened before. London’s buses have an underappreciated role in the history, of medical science. In the 1940s, a single study of London’s transport workers transformed epidemiology(流行病学), medicine and the way we live now. Every time you go on a run, check your step-count, or take the stairs instead of the lift, you are following a path pioneered by the feet of the workers on London’s buses.

In the late 1940s, doctors were worried. Britain was suffering from an “epidemic” of heart disease and no one knew why. Various hypotheses(假设), such as stress, were suggested; but one thing that was not exercising researchers was exercise. The idea that health and exercise were linked “wasn’t the accepted fact that we know today”, says Nick Wareham, a professor of epidemiology at Cambridge University. Some even felt that “too much physical activity was a bad thing for your health”. Navvies, miners and farmers who did physical exercise also suffered from various diseases and died young.

At this time a young doctor called Jerry Morris started to suspect that the excess deaths from heart disease might be linked to occupation. He began studying the medical ‘records of 31, 000 London transport workers. His findings were breathtaking: conductors, who spent their time running up and down stairs, had an approximately 30% lower possibility of disease than drivers, who sat down all day. Exercise was keeping people alive.

Morris’s research was eventually published in 1953, just three years after a study by Richard Doll proving the link between smoking and lung cancer. Morris’s work had consequences both big and small. Morris now also took up exercise, handing his jacket to his daughter and just running. “People thought I was bananas.” Slowly, the rest of the world took off its jacket and followed.

How often do you exercise? A new study found that most kids aged 12 to 15 weren't getting enough physical activity. The results were based on about 800 kids. As part of the study, the kids tracked and reported on their own activity levels, and took physical exams.

U. S. fitness guidelines suggest an hour or more of physical activity every day. According to the study, only 1in 4 U.S. kids get enough physical activity.

"It's certainly worrying to see that our kids have such a limited amount of physical activity each day,” said Dr. Stephen Pont. He is an expert on children's health.

Few kids in the study met the guidelines on physical activity that raises the heart rate and makes people breathe harder. Overall, about 25% said they got an hour of that kind of exercise every day. Kids also reported on which activities they did most often outside of school gym class--basketball for boys and running for girls.

The study found that fat teenagers were less active than normal-weight girls and boys. Fat girls were slightly less active than normal-weight girls, but levels were similar among overweight and normal-weight boys. The study also said that the percent of fat children aged 2 to 19 was 17%, or about 12.5 million kids.

"There's always room for improvement," said Tala Fakhouri, who was the leader of the study. She also said the results provide useful information to help with fitness campaigns such as Let's Move, which was initiated by Michelle Obama in 2010. To inspire kids to eat right and get in shape, Michelle visits schools and holds exercise events. She also calls on schools to offer regular gym classes.

The study also found kids who get physical education at school may get better grades.