More and more people will need to sleep outside. Already there is an entire population that does. Have you seen Oregon, Portland? The entire west coast of the United States is littered with the homeless actually. It's the weather out there. You can survive outside out there all year really, out west in the States in spots.
Regardless, sleeping outside is camping anyway. And people all over camp on the weekends all the time, or for a week or something. To camp is simple afterall. The less junk you have, the better you are. Camping without a tent is not so difficult as you might imagine. With only any old backpack packed, you can camp comfortably enough without a tent.
A sleeping bag is a nice thing to have, for camping in general—a requirement I'd even say. There's an entire industry out there. I've slept in some sleeping bags that are nicer than beds. But I've paid for the luxury, and handsomely. It's well worth the cost however in some cases. But you can find very good sleeping bags for very cheap too. I suggest those Army MSS bags. That they're filled with synthetic insulation is the most important thing, polyester—versus feathers—down feathers, more specifically. Camping, it's nearly impossible to avoid moisture completely, especially camping long-term. Polyester stays warm when wet. A polyester, synthetic-filled sleeping bag is your best bet to have for camping—and your cheapest option, generally.
If a sleeping bag can't be had, any chunk of polyester or wool fabric will do nicely to wrap yourself in and sleep. Avoid cotton at all costs, cotton sheets, cotton comforters—cotton clothes. Cotton sucks heat away from you. It keeps you cool on hot days. But for cold, wet nights,—the worst, you should always expect—it could kill you, cotton—if you're not wrapped in something else, nylon or something similar, to trap that heat.
So you've got your polyester, a sleeping bag or a chunk of fabric. Next you need to be insulated from the ground on which you'll sleep. Or else the ground will steal your heat, the natural neutralizer that is dirt. Yoga mats work. Most any foam mat will work generally actually, closed-cell or closed-form, I've seen them called—no holes. Again, there is an entire industry out there—camping gear. There are blow-up mattresses available, very small to carry, very valuable, easy to deploy. These are expensive, but nice, so you get what you pay for the most part. An alternative to that, to paying anything, is cardboard. Corrugated cardboard, most specifically, the one with the zig-zags between the layers. This can be got almost anywhere, recycling bins outside of stores or homes, for nothing. And it works just as well as anything to keep you insulated from the ground. It even bounces heat back up to you as you sleep, it seems like to me—for which you pay a pretty penny in some of those nice, compact, blow-up mattresses.
On nice nights, that's all you need to sleep, insulation from the ground really. But rain is unavoidable. So you need a tarp in case, a sheet of nylon or something similar to nylon. You can string-up the nylon like a tent. Which requires only a tiny bit of skill. You must choose a particular spot to sleep, one preferably properly spaced between trees—pine trees the preference, for the needles below which soften the ground a bit. And you must know some knots to tie the tarp above you correctly—timber-hitch, clove-hitch, trucker's-hitch, double fisherman's knot, prusik knot, the bowline, the taut-line-hitch. Some rope you'll need too then. Parachute chord works nicely. It's cheap, easy to find, and it's strong. But any kind of chord will work really, whatever can be had. Or else in a pinch, you can just simply wrap the nylon directly round you in the sleeping bag or chunk of fabric. It'll keep you from getting too soaked from the rain. But the heat from your body—which is the best heat of all, the source of heat actually, your body heat—will create some condensation trapped like that, no way to escape, wrapped in nylon. On extremely cold nights, the nylon wrapped directly round you in the sleeping bag or fabric can keep you warmer. But you must watch that condensation. Wet and cold kills.
To carry 2 sheets of nylon is convenient. It adds barely any weight to your backpack, and it steals barely any more room. You can always make a hammock, too, with the extra sheet of nylon, which requires only a bit of skill again, but can be very practical in a lot of rain, to get off of that wet ground. Some stronger rope I'd recommend for the hammock then—some Amsteel Samson rope, and some webbing for round the trees.
Fires of course are very nice too. Not everywhere you can make them however—in some cases. But when you can, even just for the added psychological benefit, I suggest having a fire near you while you sleep, to help keep you warm—for light in the dark. With fire alone really, and quite a bit of skill making some shelter from debris, you can comfortably sleep outside without a tent. But with a sleeping bag, or some polyester or wool cloth, insulation from the ground, and some nylon, you can almost anywhere sleep comfortably enough outside anytime—and all for no more than $50.