CAN'T be bothered to tell your Facebook friends what you are up to? A smartphone app called Jigsaw can help.

Jigsaw figures out what you are doing by monitoring your phone's microphone, GPS and accelerometer for patterns characteristic of routine activities - and it could be set to send the results to social networking sites.

More importantly, Jigsaw can log how active you are each day, producing records that could be useful to a doctor or fitness trainer. Its pattern-recognition algorithms can identify a range of behaviours, making its logs more detailed than those of similar apps, says Hong Lu at Dartmouth College in Hanover, New Hampshire, who developed the app in collaboration with the Nokia Research Center in Palo Alto, California.

For example, the jolts produced when the user is walking depend on whether the phone is in a trouser or jacket pocket, so the software can recognise both patterns. Information from the other sensors helps to further define the activity.

The less active you are, the less often Jigsaw kicks in, so it can capture your activity patterns while minimising the drain on the phone's battery. "If you're stationary there's no point in getting regular GPS readings," says Lu. He adds that a smartphone running Jigsaw in the background would get 12 hours of use from the battery, so you can log your behaviour from morning till night on a single charge. "Without smart power management it would drain the battery in 6 hours," Lu says.

Jigsaw does not send raw data to a server, so the amount it can store is limited by the phone's memory. This was deliberate, to address potential privacy concerns, says Lu, who presented the work at the ACM Conference on Embedded Networked Sensor Systems in Zurich, Switzerland, this month. The app, due for release next year, runs on Apple's iPhone and Nokia smartphones.