Temperature Variance in Small Greenhouse.

[PaulTandberg]

I have a small, 8' X 8' poly fabric (opaque) greenhouse (similar to material used for high tunnels and quickie car garages). It will get warmer than the ambient air temperature during a sunny day. I understand that. Conversely, it often gets colder than the ambient air temperature during the night (sometimes by as much as three or four degrees). I don't understand that.

I also understand that this is something that is not uncommon with small greenhouse, at least of a certain type.

What is it? Why is it? And is there a simple fix (like a small fan circulating air or opening it up at night?)

Thanks