Damp bedroom floor.

My daughter had some friends stay over sat. night and one of them slept in a sleeping bag on a plastic coated exercise mat on the floor.
When i was taking up the mat yesterday morning the underside of the mat and the floor under it was sodden with condensation. It's a timber floor on a concrete screed which is down about 3 years and we don't have underfloor heating.
Just wondering what caused this, I haven't noticed any dampness in the room before?

the person in the sleeping bag acts like a heat source....
the plastic exercise mat acts like a vapour check layer.. it doent allow moisture through it...

condensation is formed when warm element touches a cold element.... this can be air, a solid etc... the warm element causes condensation on the cold surface...

in this case the person heats up the bag and plastic mat... the cold surface underneath, both the timber and concrete contain moisture.... they are never completely moisture free....

the heat is transfered from warm to cold (it tries to equalise) therefore condensation forms on the underside of the plastic mat...

Thanks syd, makes sense.