Aluminum Macbook randomly falls asleep

Hi there,

I have a new aluminum macbook. I have it plugged in to an external display. Recently, a new problem has popped up. I'll be doing something (watching a video, typing, anything), and it will suddenly fall asleep. After it falls asleep, keyboard or mouse inputs don't do anything. I have to close the lid, wait a while, then open it back up for it to wake up. Any ideas on how to fix this?

Thanks!