Birds singing and chirping .Soothing birds sounds. Sounds for cats.

The effect of birdsong on our brains is being researched by Eleanor Ratcliffe. She is carrying out a three-year study looking at how natural sounds can improve mood and attention for her PhD in Environmental Psychology at the University of Surrey. The research is part funded by the National Trust and Surrey Wildlife Trust.

"We need to find out if, how and why birdsong can be of psychological benefit. Many people believe it is," she says.

In the science world it's known as postprandial somnolence but in the everyday world it's commonly known as the post-meal slump.

It's the period after a meal when blood sugar levels drop, causing you to get sleepy.

As an office bed and sanctioned afternoon naps are not likely solutions, researchers are looking into what can lift people out of this soporific state after lunch. Birdsong has been found to work, they say.

An experiment at a primary school in Liverpool - run by "sonic branding" company Condiment Junkie, Glyndwr University and architects Nightingale Associates - found that playing pupils a soundscape of birdsong and other natural sounds made them more alert and better able to concentrate after their lunch break.

The key to the most effective audio was lack of repetition, say those involved.

Birdsong works because it's stochastic, made up of lots of random sounds. There is no repeating rhythm or pattern to focus on.

"It doesn't get stuck in your head and annoy you but it doesn't lull you to sleep and bore you either," says Russell Jones, from Condiment Junkie.

"I'm not sure there is any other sound that can do what birdsong does. It should be part of the soundtrack to everyone's day."