I'm a bit embarassed to say this, but I had the same problem with a job and dealt with the sudden-onset fatigue in two different ways. One was to go to my car on my break and lie down and rest in the back seat. I set a timer to wake me up in ten minutes. This was a little embarassing when someone caught me doing it, but then we both laughed when she said she'd done the same thing in the past!
The second way was to just go to a one-person bathroom and lie down on the floor in complete darkness for a couple of minutes. It removed the light, the sound, and the presence of other people completely. I would do a few breathing/meditation exercises, then got up, turned on the light, flushed the toilet (!) and left.
I did find that I built up a tolerance to all the stimulation over time, but I also learned to NEVER plan evening activities on the days that I worked. Working 5 days a week didn't allow enough down time for me, so I first arranged to work four ten-hour days with Wednesdays off, so I had a recovery day in the middle of the week. Then I cut my hours down to thirty hours a week when I'd gotten some debts paid off, as the money wasn't as vital. That schedule worked VERY well for me.
My advice is, be creative and look for any way possible to "disappear" from the stimulating environment even briefly, and then make the most of those precious few minutes. Good luck!