I remember working at Movie Buffs back in my college days. It was a 24 hour video store on 400 South at about 5th East in Salt Lake. The building previously had been used for a Chinese restaurant.

Anyway, I regularly worked a midnight to 8am shift and drove up the road to class when the 8am person arrived. Sometimes, business wasn't too busy but it only took one customer in the store to ruin everything.
By "ruin" I mean waking me up.

I stapled a thin rope to the top of the old wooden door and ran it along the ceiling through the lights all the way to a secluded hallway where the restrooms were. I then tied the other end of the rope to a heavy broom that was propped into a standing position by the tension of the rope. If anyone opened the heavy front door, the broom would crash. I positioned random metal objects around the area where the broom would fall and I had a perfect alarm. I set myself up with a couple of blankets and a pillow and I was set.

Whenever anyone opened that door, CRASH!!!, and I'd jump right up. I'd stash the blankets and pillow in boxes outside the door and run to the front as if I'd been elsewhere in the store re-shelving movies or whatever.

Inevitably, customers hardly ever gave me much of a break. One would come in and wander around for 30 minutes and as soon as the selfish 2am video shopper left, another one would walk in behind him.
Even worse was when homeless people would pop in during the winter to get out of the cold. Griping at them to get out of the store so I could have some nap time was tempting, but I didn't have the heart to do it.

Customers and pretend customers suck.