'Putting clocks forward boosts kids' exercise'

Behind the Headlines

Friday October 24 2014

Child playing as the sun sets

Playing outside in the evening can be healthy fun for children

“Moving the clocks forward by one extra hour all year in the UK could lead to children getting more exercise every day, say researchers,” reports BBC News.

In the UK, the clocks move forward one hour during the summer months so that there are more daylight hours in the evening (daylight saving time).

A new study has found that children are more active in daylight saving time, regardless of the weather conditions. Its findings suggest that if the clocks were moved forward by one additional hour all year round it could lead to each child in England spending an average of 1.7 extra minutes in moderate to vigorous physical activity per day.

The end of summertime

This Sunday, clocks will go back an hour at 2am

While small, the extra exercise isn’t trivial because children only engaged in this level of activity for 33 minutes a day. Also, spread across the whole population, the impact could be considerable.

However, the study couldn’t eliminate the possibility that there are other things responsible for the associations seen.

The estimates of how much children’s activity would increase would only hold true if the only reason they aren’t more active normally is daylight. Although the researchers took into account the weather in their analyses, it is difficult to adjust for expected (rather than actual) weather conditions. For example, school sports days tend to occur in the summer, in the hope that it will be warm and dry.

 

Where did the story come from?

The study was carried out by researchers from the London School of Hygiene and Tropical Medicine, the University of Bristol and Bristol Biomedical Research Unit, and the International Children’s Accelerometry Database Collaborators. The International Children’s Accelerometry Database is funded by the UK National Prevention Research Initiative. The researchers were funded by the National Institute of Health Research.

The study was published in the peer-reviewed International Journal of Behavioural Nutrition and Physical Activity. This article was open access, meaning it can be accessed for free.

The results of the research were well reported by the BBC and The Daily Telegraph.

 

What kind of research was this?

This was an observational study looking at the relationship between daylight in the evenings and the amount of activity children undertook. Researchers used information from the International Children’s Accelerometry Database, which contains activity data from children participating in studies worldwide, collected using motion sensors called accelerometers. These sensors are worn on the waist, and objectively measure physical activity levels without needing to rely on people remembering and reporting how active they were.

The researchers wanted to assess whether:

  • more evening daylight is associated with higher total physical activity, even after taking into account the weather conditions
  • the overall differences in physical activity in different times of the year are greatest in the late afternoon and early evening
  • changing the clocks affects activity levels

As with all observational studies, it is not possible to prove without a doubt that changing the clocks is directly responsible for the differences in activity levels seen, as other factors could be contributing. However, given that it would not be feasible to test the effect of changing the hours of daylight in a randomised controlled trial, this is the only way of assessing the potential link.

 

What did the research involve?

The researchers examined accelerometer data from 23,188 children aged between five and 16 in nine different countries. They looked to see whether physical activity varied according to time of sunset.

They then examined accelerometer data from the 439 children who had data from a school day just before and just after a clock change (within a week).

The researchers adjusted their analyses to take into account differences in weather (amount of rain or snow, humidity, wind speed and temperature), and differences in the characteristics of the children (such as age, sex and weight).

 

What were the basic results?

Longer evening daylight was associated with a small increase in children’s daily physical activity, even after taking into account other factors. When sunset was at 9pm or later, children spent approximately six minutes longer in moderate to vigorous physical activity than when sunset was at 5pm or earlier. The average time spent performing moderate to vigorous physical activity was 33 minutes per day, so a six-minute difference is not as trivial as it may seem.

The differences in physical activity were greatest in the late afternoon and early evening. The researchers found no association between activity levels in the morning and hour of sunset, and generally no association for activity in the early afternoon. This supported the argument that the extra hours of evening daylight were directly causing the increase in activity seen.

These associations were also seen when comparing the same child just before and just after the clocks changed.

The associations between hour of sunset and more physical activity were only consistently observed in children from five mainland European, four English and two Australian samples. The link was not consistently seen in American, Madeiran and Brazilian samples.

In the English studies, each additional hour of evening daylight was associated with 1.7 extra minutes of moderate to vigorous physical activity per day.

 

How did the researchers interpret the results?

The researchers concluded that in Europe and Australia, more evening daylight appears to increase children’s physical activity. They say that although the average increase is small in terms of minutes per extra hour per child, these small increases would add up when applied across all children in a population.

Also, the increase “compares relatively favourably” with the typical increases in physical activity that can be achieved with intensive programmes aimed at getting children and adolescents to be more active. They conclude that “the introduction of additional daylight saving measures could yield worthwhile public health benefits”.

 

Conclusion

The current study has found that more evening daylight is associated with increased physical activity in children, even after taking into account the weather. The results suggest that if the clocks were moved forward by one additional hour all year round, it could lead to children in England getting an estimated 1.7 extra minutes of moderate to vigorous physical activity per day.

Although this seems like a small amount, it is not trivial in relation to the average amount of activity children were found to have in a day (33 minutes). If every child in the country has a small increase in activity, this adds up to a considerable total increase.

The study’s strengths include the large total number of children assessed, the fact that they used an objective measure of activity, and that the children came from a range of countries.

There are some limitations to the findings of this study. The data was mainly cross-sectional, and it is difficult to eliminate the possibility that there are other things responsible for the associations seen. The estimates of how much activity could increase with a clock change assumes that the differences seen in activity were completely due to the extra hour of daylight. The researchers also note that although they adjusted for the actual weather, it is difficult to adjust for expected weather conditions. For example, school sports days tend to occur in the summer in the hope that it will be warm and dry.

Given that a randomised controlled trial to assess the impact of a clock change would not be feasible, this type of study is likely to be the only way to look at how daylight hours affect activity. The rising levels of sedentary lifestyles and obesity worldwide mean that finding ways to increase physical activity is an important policy area. While this study will contribute to the debate about whether the clocks should go forward an extra hour, there are likely to be a range of other factors the government will consider in making a decision – such as potential business and economic impacts.

 

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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