I would never drink a prep late at night. It can take many hours for me to finish 'going' from a prep and then my whole night is ruined (it's happened to me before...).
In the past I was completely cleared out from the first half of the prep, in fact I ended up vomiting the second half in the morning because I already felt so weak so it didn't do anything for me anyway. I think you'll be fine.
dx'ed UC pancolitis 5/12
past meds: asacol hd, VSL#3, apriso, rowasa, xifaxan, 6mp, cortifoam, pentasa, cimzia, canasa, butyrate, flagyl, cipro, prednisone, remicade, methotrexate, cholestyramine, cortenema
current meds: none!
step one: colectomy, end ileo 1/16/13
step two: j-pouch construction, loop ileo 5/1/13
step three: takedown 7/31/13