I've read that hammocks are a very light way to travel through the wilderness. You just need the hammock, a cover, and a light sleeping bag, but there must be disadvantages compared to taking a tent or a bivy sack. What are they?
Hammocks are cold. The weight of your body compresses the clothes or sleeping bag, and air circulates underneath you, as opposed to a tent where you usually have a pad and the ground for insulation.
It seems like it would be tough to stay dry in the rain in a hammock. It's nice to have a tent to get into in the rain in between hiking/playing and sleeping if it's raining.
I've been a hammock backpacker for about three and a half years now. I love it. There's not a better way to backpack in the summer in my opinion. But despite numerous advantages to hammocking, there are some downsides (tradeoffs):
The biggest potential downside, as others have mentioned, is heat loss from the underside of the hammock, something my friends and I like to call "CBS" (cold butt syndrome). Insulation from clothing and sleeping bags is compressed and that limits the temperatures in which you can hammock comfortably with a sleeping bag. Adding a sleeping pad under you works fairly well but is not ideal because it adds additional weight (that you are trying to eliminate by hammocking.) Underquilts are the ideal insulation for cool-weather hammocking, but they are usually homemade or relatively hard-to-find compared to sleeping bags. Also the "bridge effect" comes into play when there is even the slightest breeze. The air moving below and above the hammock can cool you drastically, even in the summer. (That said, hammocking is perfect in hot weather.) Obviously this limits hammocking to primarily warmer months unless you have a bomb-proof hammock setup for winter (some do). However when you get to that point you may as well have a tent because any weight savings by using a hammock setup are essentially nullified by the extra gear and insulation needed (there are exceptions).
You can potentially get very wet if you don't learn to properly set up a good tarp or rain fly system over your hammock. However this is only at first. Once you get wet you learn really quickly. I've weathered some really nasty storms in my hammock and stayed completely dry with several different types of systems. Some are better than others. There is an art to a good tarp setup and so many ways that I won't go into it here. A good tarp set up will also the alleviate the problem of water running down your lines onto the hammock.
There is a learning curve. To get good and comfortable with regular sleeping in a hammock, then you have to be willing to change some habits and learn new ones. And there's all kinds of new gear to research and learn about (not a downside for me, but for some). So you have to learn what type of setup works best for you, and there are many to choose from.
You lose a certain amount of privacy. When tent-camping you can just get inside alone and change clothes with no one looking. When one only has a hammock with a tarp over it and no walls, then one must get a bit more creative to conceal one's man-parts or woman-parts when changing. Obviously at night this doesn't matter very much...unless your friends tend to pack night vision goggles. This one really isn't a big deal once you figure out the tricks, or you may not care.
Sharing a hammock overnight isn't really an option for most people, especially for a multi-day trip. If you try, it had better be someone you are willing to be up close and personal with all night long. Even the double-sized hammocks squeeze two people together, which just isn't comfortable for an extended period of time. It's also impossible to not disturb the other person in the hammock if you move at all or get out of it. Sharing a tent or a tarp-tent isn't nearly as hard to do.
Weight savings can actually be negated with multiple people. Consider this example: three people sharing the load of a modern lightweight or ultra-light 3-person tent (footprint, body, rainfly, poles, stakes) vs. 3 people carrying their own individual hammock set-ups (hammock, straps/rope/webbing, carabiners, tarp, stakes, no-see-um netting for some people too). Now this depends highly on the tent model and the type of hammock setup, but generally speaking you will likely save weight by splitting up the tent among all three people, especially if you are already using a sleeping pad in your hammock. A shared tarp-tent could mean even more weight savings. (There are ways around this dilemma. You can sometimes share a large tarp with several hammocks if the trees are arranged nicely.)
You aren't guaranteed to find good trees for your hammock at every campsite. This has certainly been a challenge at some places I've camped. If you doubt that the area you're visiting will be good for a hammock, then don't depend on the hammock as your primary shelter. Even if it's rocky, frozen, snow-covered, just sand, or other difficult terrain, you always have ground to set up a tent.
It's just plain harder to go to the bathroom at night! You get better at it with practice, but sometimes it is simply a pain to climb out of your sleeping bag and hammock, go relieve yourself while half asleep, and then try to wiggle back in to your hanging bed in which you were so, so comfortable. Plan ahead. Don't drink too many fluids before you go to sleep in a hammock.
In a lot of places it will be quite difficult finding two trees the right distance apart to hang it. When it's late and you've been walking all day, all you want to do is lie down and rest, you might spend quite a long time looking for those two trees.
Here in the UK when it rains, it often comes horizontal, so even with a tarp over you, you're going to get wet. And maybe motion sickness if it's windy!
I am relatively new to hammock camping, but the only downside that I am aware of is possibly not being able to find a place to hang. As others have mentioned, you do have to account for heat loss out of the bottom of the hammock, but that can be solved simply/cheaply by putting closed-cell foam pad in the bottom of the hammock (generally more expensive is the underquilt). Be sure to check out http://hammockforums.net . There is a great community of hangers there, from backyard and bedroom hangers to ultra-lightweight backpackers.
My experience with hammocks: serveral multi-day bike trips.
You need trees. Depending on the area this can be a problem. I have found myself looking desperately for trees in Hungary. On the other hand you may find spots to tie your hammock in strange places (abbandoned customs station, bridge)
The rain can be an issue, but that what your tarp is for. Also note that water comes down the lines of the hammock. Use a carabiner at the connection between the lines and the hammock.
Stormy weather can also be an issue. The problem can again be migrated with a change of location or a tarp.
Your sleeping bag will be compressed and you can get a cold butt. My solution: My hammock is double-walled and i put an insulated camping blanked between the lower walls.
Moskitos. Use a hammock with a mosquito net. Trust me.
Getting up at night can be uncomfortable (tall grass, slopes, dense bush)
Now the good parts: Space an weight of hammock and tarp are the same as a smaller sleeping bag. Setup within minutes, minimal footprint, sleeping on any slope trees grow, camping out of sight 10 meters next to a road. No worries about stones and roots below you.
Hammocks are great for hot weather. I have backpacked with a hammock in the Virginia summer, when the nights were 85°F-95°. There are certainly enough trees here, and in any densely forested area. It is actually preferable (with a mosquito net) because you stay cooler. When it rained, I slept on the ground with a small rain fly, because water likes to come down from the attachment points. Bivys pack so small that if one were concerned about the weather, you could pack both in less space than a full sized tent.
I had the same worries as the OP, but a buddy of mine brought a store-bought hammock on a 3 night trip a few years ago. I started investigating and found this site. I made my own for a few bucks (< $20 for the ripstop nylon, a few bucks for some rings, a few more for straps, paracord and a couple carabiners), have used it for 2 years now, and it's still going strong.
The guy talks about staying dry (use a tarp), staying warm (he's got pictures of sleeping in several inches of snow), and keeping the bugs off, too. I've never been cold in mine, but as Chad mentions (+1) it's easy to add an underquilt, if you like, and that's what I'd do if I were going to be somewhere below 40F. I think Simon (+1) has a good point about finding the two trees, but you only need one (1) tree, and a sturdy stick of the appropriate height. (?!) You attach one end to the tree, then run the other cord over the stick/post then into the ground, and the hammock stabilizes itself (saw this somewhere that I can't quite find right now. Try it!).
In short, after 20+ years of packing a tent around (and I tried a bivy once, but it was too cramped for me), I don't see myself taking anything other than a hammock for the forseeable future. Your mileage may vary.
By the way, I've use the nylon hammock, an ordinary sleeping bag, and one of those egg-crate sleeping pads. Just got a Therma-rest pad (the most expensive link in the chain) that I haven't tried yet.
One downside I've recently found out is that some hammock use can violate Leave No Trace policies. If you aren't careful about the rig you use to hang the hammock, you could injure the trees. The wider the strapping around the tree, the better for the tree. I suppose this doesn't mean all hammocking is bad for LNT, but it is common to hang a hammock with ripcord or similar such rope, and that's supposed to be no good for most trees.
Some of the boy scout camps we go to mention this, although so far none prohibit use. It's also part of our leadership training. As a result, the scouts are recognizing some downsides but which can be mitigated with better equipment and training. It is interesting to note that the BSA is also promoting the concept, and some troops are reporting complete conversion to hammock-only camping.
And some interesting dialog in this forum discussing a specific ban on the subject, as well as discusses some other downsides which, like my own answer, can be mitigated with proper resources and training:
I believe you could be faced with back problems if you slept in a hammock every day of your life. I don't think it would be a problem for just a few days once in a while though.
Another problem probably worth mentioning is that, unlike a tent, you can't use your hammock (even with a tarp) to keep your gear out of the weather. Unless you bring your whole pack in the hammock with you, in which case you probably won't be very comfortable...
My personal problem with hammocks in general is my hips aching after an hour or so. I sleep on my side when in a bed, instead of my back, meaning I wouldn't be sleeping in a comfortable position on a hammock, thus preventing sleep and then tiredness in the morning. Camping provides no guarantee that you will find a place long enough to put a hammock that doesn't fold in on itself, either, leading to hip and back problems. Stiff hips would make the hiking harder the following day. If having your feet elevated to reduce swelling is more important than sleeping on your side, then it may not be as bad of an idea.