I used to sleep under the stars (no tent or sleeping bag) while camping in the everglades and other areas of florida. I also would in certain parts of Texas. Never had a problem. In western Texas and US I will use a tent, since the colder climate will make your body attractive to snakes, who seek out warmth. Fortunately, the rattlesnakes in that area are timid and avoid humans. That isn't the case for all snakes. The copperhead, a southern species, is very aggressive. I have been "charged" several times by such snakes when canoeing, or encountering while hiking.
You best bet is to contact a herpetologist at a local university and ask what the behaviors for such animals are in your area and what their suggested precautions are.