This little 'dirty secret' of the (larger commercial) hammock industry was quite a little shock when I first slept in my hammock, on a 'warm' summer eve. The initial draw for the uninitated is reduced weight compared to your tent, and the ability to leave your (heavy, bulky) pad home and still have superior comfort. That's how I got 'suckered' into this hammock thing.
If the under insulation isn't just right, and you're not a 'hot' sleeper, it makes for a particularly woeful night. At what temp things get woeful is pretty individual, but, on average, it seems to start around 70*F for many of us. At 50-55, you'll be very likely to feel cold.
(edit: this entire paragraph deleted due to redundancy of other posts.)
I've had pretty good results at warmer temps hanging a cheap ($7) fleece blanket under the hammock, by tying elastic cord to the corners, similar to a Jacks R Better type UQ suspension. The difficulty with this is closing the sagging gap at the top and bottom, where you'll feel the chill. Quickly sewing a hollow hem at the top and bottom, and passing the elastic through it, would be a possible quick solution. A single, thin (cheap) fleece blanky probably won't add much insulation, though.
I ended up with a JRB nest, but I must confess that I hate the weight/bulk of all effective under-insulation solutions compared to the simplicity and effectiveness of a simple foam pad on the ground.