using a plastic ground sheet is traditionally used under the tent floor, outside the tent, to protect the tent floor from sharp objects, abrasion etc that wears out a floor and makes holes that water under a tent will come through into the dry tent inside.
Putting a plastic sheet inside your tent will do nothing to protect your tent floor from sharp sticks, rocks or whatever, and holes from these things will insure that water gets inside your tent.
Having already stated to use a plastic sheet under the tent, I didn't feel it would be necessary to repeat it. However, a plastic sheet inside the tent, bathtubbed up the walls slightly, has saved many tent floors(the first thing to go bad in a tent) and kept many a sleeping bag dry in severely wet conditions.
I've been doing this for over 50 years, and have learned a few things about camping in a tent, and sleeping on the ground. Just trying to pass some along.