Sleeping out under the stars is a wonderful experience - especially if you don't need to take your eyeglasses off to sleep (darn!). But it takes some getting used to.
I've slept in my sleeping bag with just my sleeping mat and inexpensive tarp underneath as a groundcloth. The sleeping bag and the light clothes I wore were all the "shelter" I needed. This was usually at about 5,000 feet in the Sierras.
I tend to worry about snakes and predators (the two and four legged kind). But I've never been disturbed and neither have my friends who do it more often than me. If you try it for the first time, get some buddies to do it with you. This will assuage your concerns and it will be a lot more fun.
- Woodsorrel