The reason I hope this is okay is because I am too tired and don't have the time to clean the brush versus keeping it in the water.
Thought is may be too late for you, when I am in this situation I put the paint brush in a zip-lock bag, get as much air out as I can and seal it up. I have not yet had a problem with brushes left for several hours up to over night. Sometimes I don't have big enough bags, so I leave the handle sticking out, roll the bag up to keep the plastic tight against the brush and paint and then put tape around the handle to help keep it sealed.
The worst time for me - This March I was painting cabinets and DH was cutting boards in the shop - He opened the house door and hollered "We have to go to the hospital NOW!" Four hours later we were back home and his finger was sewn back together - and I finished my painting.
I wet it slightly and wrap the bristles tightly with saran wrap or even a plastic bag.