SAN FRANCISCO, US - Google has adapted its free email service to help those letting loose after a few evening cocktails or succumbing to lovelorn moments from firing off messages they might regret in the morning.

Mail Goggles software comes to life after dark and on weekends, when altered states of mind are more probable, and requires that five simple math problems be answered correctly in less than a minute in order to send a Gmail missive.

"Sometimes I send messages I shouldn't send," Gmail engineer Jon Perlow wrote Monday in a website posting announcing the optional new feature.

"Like the time I told that girl I had a crush on her over text message. Or the time I sent that late night email to my ex-girlfriend that we should get back together."

Gmail users can adjust their email settings to activate the Goggles feature and dictate the times it is active.

"It will check that you're really sure you want to send that late night Friday email," Perlow wrote.

"And what better way to check than by making you solve a few simple math problems after you click send to verify you're in the right state of mind?" --AFP