If take miralax (polyethylene glycol) close to bedtime will my body he enough water to keep it working until next morning?

Presumably . . .: Miralax, a brand of generic polyethylene glycol 3350, is used as an osmotic laxative to treat constipation (http://www.drugs.com/miralax.html). It works by increasing water in your bowels. Typically, this should not lead to dehydration. If you're concerned, take it in morning upon awakening. Better yet, discuss your concerns w/prescribing physician & dispensing pharmacist.