I do overnight runs on the DAKO stainer all the time.  I put a
shallow tray of water in the bottom to keep things moist, in Colorado
the slides dry out quickly, I also have a UPS (uninterupted power
source) unit hooked up to the computer so that if the power should go
out the computer will stay on.  Be sure to switch your haz waste back
to reg. waste so you use water instead of buffer every hour to wash
the slides.  What else do you want to know?
Patsy Ruegg