I was just wondering because, my tent will have a big wet spot in the morning where my air mattress was.
I’ve noticed the same thing. It’s most likely condensation. When the ground is cold, the tent floor will be colder than your air mattress, causing moisture to collect between the two layers.
You could put a mat under your tent, I suppose–this would solve the problem. But I wouldn’t recommend it because of the expense, extra weight, and difficulty involved in setting it up.
I think it’s easier to just mop up the moisture with a towel or spare piece of clothing.
But that’s just my opinion. If you’re getting a lot of condensation and it really bothers you, it might be worth putting an extra pad underneath.
If you decide to give it a try, a thin, inexpensive foam pad like the Wenzel Closed Cell Camp Pad should do the trick.