It can be hard to find motivation to exercise, but new research suggests it could be as simple as flicking a switch in the brain.

Scientists in Texas have accidentally found a way to get mice off their little mice couches and into the running wheel, and think it might work in humans too.

They were testing whether a gene called Dnmt3a was linked to weight. By turning it off, they expected the mice to gain weight.

"Somewhat disappointingly, however, the Dnmt3a-deficient mice were only slightly fatter than those that were not deficient," said study author Harry MacKay.

Investigating further, they found the mice with the gene turned off ate no more or less than those with it turned on. So how did they end up slightly fatter?