From NBC News:
Even if we don’t realize we’re doing it, adapting or changing the accent we grew up with takes effort, both cognitive power and motor control. You know that when you’re drinking – or tired or sick or very cold, for that matter – it’s harder to make your body move in exactly the way you’d like it to. That applies to our mouths, too, and that starts to show itself in our speech.
“We slur our words, and it’s harder to maintain the motor coordination and control needed for effective fine motor execution needed for speech production,” explains Amee Shah, director of Cleveland State University’s Research Laboratory in Speech Acoustics & Perception. Our brains, of course, are affected, too, when inebriation – or, again, illness or sleepiness or being cold – sets in, which means all of our available brain power is devoted to simply managing the easiest tasks. “Anything more than the simple gets affected, as we just don’t have enough cognitive resources available for it,” Shah explains.
Pro tip: PLAY UP THE ACCENT WHEN DRUNK. Girls get a kick out of it.