Interview: Dr Anna Goodman, London School of Hygiene & Tropical Medicine

What's the background to the research?

In England and Australia in particular there has been much debate about whether we should change the clocks to shift them forward by an extra hour each year. Since 2010, Parliament has considered proposals that would have led to British children seeing about 200 hours more daylight hours a year and to adults seeing about 300 hours.

Australian states have held referenda on the issue and there was a political party set up in 2008 called Daylight Saving for South East Queensland. People have often thought that putting the clocks forward an extra hour would help children to be active and we decided to try and quantify it.

Tell us about the study.

Twenty-three thousand children aged five to 16 years wore devices called 'accelerometers' around their waists to measure body movement during the day until they went to bed.

We analysed data on children in England, Australia, the US, Norway, Denmark, Estonia, Switzerland, Brazil and Madeira. We were able to make comparisons throughout the year.

What were the key findings?

The strongest effects of the extra hour of daylight were in England, mainland Europe and Australia, in places where it was not too hot to go out and enjoy it. We found children's total daily activity levels were 15-20 per cent higher on days when the sun went down after 9pm, compared with winter when it was before 5pm. This equates to a 5 per cent average increase in physical activity per extra hour of evening daylight, for every child in the country for every day of the year.

This figure of 5 per cent may sound modest, but lots of modest steps in the right direction make a real difference to combat a lack of physical activity. It is also important that this increase was seen in boys and girls, across children from richer and poorer backgrounds, and across both overweight and healthy weight children. It wasn't just the sporty ones; it was a range of children.

What would be the main benefits of putting the clocks forward an extra hour all year round?

This is the strongest evidence to date that in Europe and Australia evening daylight increases physical activity in the late afternoon and early evening - the 'critical hours' for children's outdoor play.

There are many good arguments in Britain for being on the same time zone as Spain and at least having a trial of that. The ideal would be one hour ahead of GMT in the winter and two hours ahead in the summer.

Dr Goodman was speaking to Catherine Gaunt.