If a lightning storm is coming, and you had a tent and a hammock/tarp with you, which would you choose, based only on relative safety from lightning?
Camp Selection The location of your camp will likely be a much bigger factor than what style of bedding you use. Find a low point away from any large trees and away from anywhere that may accumulate water as it lighting typically accompanies rain would be priorities. Mountain tops are great for view but are scary as hell during electrical storms.
Tents vs. Hammock In a lying sleeping position you are exposing a lot of surface area to the ground (or nearly to the ground including sleeping pad, tent, sleeping bag etc.). Anecdotally there seems to be a lesser chance of injury during a nearby strike if you have minimal contact with the ground and have any form of insulation even sandals. So in the tent on a pad, on a bag is better than lying directly on the ground but better to be a squat than lying down.
In a hammock you aren't on the ground but you are directly attached to a tree which is tall and has lots of "sharp" leaves and branches which is great for attracting lighting strikes. If the lighting hits the tree you are in you will be hit. I think if there is a nearby strike you will be better off than on the ground though.
After optimizing campsite selection I think the best you can do is be in a tent with as much under you as possible and in a squat until the storm passes. It's rare to get lighting for hours on end.
Whatever you are tying the hammock to is obviously sticking up a bit. Let's assume trees. Things that stick up are more likely to be hit by lightning. Since the hammock needs to be tied to two things that stick up, one would be hit by the lightning and the other not. The hammock is now a conduction path for some of the lightning to connect to ground via the tree that didn't get hit directly. You don't want to be in that hammock.
You may feel you are more immune to ground current by being up off the ground, but not necessarily so. If the ground current from a nearby lightning strike runs parallel to the line between the two trees, then the two hammock supports could be at substantially different voltages. That voltage would be applied to the hammock. Again, that's not good for you in the hammock.
The main danger in the tent is from ground currents. Since you have the option of using a hammock, there are taller things than the tent around. That's good for avoiding a direct hit. However, the current from a nearby hit can still be substantial thru the ground the tent is sitting on.
A "tent" implies some sort of floor, presumably made from a insulating material. You would also usually have something dry and insulating on that floor, like a sleeping pad and a sleeping bag. Between the tent floor, pad, and sleeping bag, you have at least some protection from the voltages caused by ground currents.
So while neither a hammock nor a tent is safe in the lightning storm, the tent is the better option for minimizing the probability of getting hurt by lighting.
As always with such general questions, "It depends".
In a lightning storm, you should always get well clear of any trees. The reason for this isn't that trees are necessarily the most likely place for lighting to strike but because, when they are hit, the water inside tends to boil, exploding the tree. Being hit by burning fragments of timber will likely Ruin Your Day.
Having pitched your tent clear of trees, the most likely risk is from ground path current; the hammock has a slight advantage here, as the current must go up and down the poles to go through you, and everything between the poles is dry (if your tarp is working). In a tent, you're lying directly on the damp ground with just a thin groundsheet in between. Ground path currents tend to be much lower than the main strike current (if it was to dissipate evenly across the ground surface, then the current would be inversely proportional to the distance from the strike).
Depending on what kind of poles you use for your hammock, you may be increasing the likelihood of a direct strike, but the good news is that such a strike should go straight through that pole to ground. It might melt the pole as it does so, which wouldn't be fun. Perhaps pole caps made of old tennis balls may help protect against that.
A really bad position to pitch a hammock would be across a rock cleft - if the two lumps of rock are at different potentials, then the lowest-resistance path may well be through you. Never stretch your hammock across a gap that doesn't have an obviously better electrical bypass very close by.
Ask ten people "what is the correct thing to do in a lightning storm" and you'll get fifteen asserted answers.
Lightning can travel eight miles through the ground and shut off your heart (you can survive a hit of well over 200,00 volts- albeit with some significant exit wounds- but 0.5 amps will shut off your heart). As has been previously stated, most deaths occur from current traveling through the ground and through your body as you are in its path (it can travel in a "line" as if a bolt on the ground, or in a ripple radiating out as water when something drops on the surface). If points above and below your heart are both touching the ground when the current passes through you, it is like a really powerful AED hitting you. This is why the graphic Paparazzi provided is so commonly taught- the current will pass from one foot, up a leg, through your ass, down the other leg and foot into the ground.
Go ahead- get into get into that position. We'll wait for a minute....
How long did you last? When I was a guide in Utah we had storms come through that would last over an hour (average was probably 20 min, longest I can remember was 2.5 hours). I can't do that for more than five minutes, no way that's realistic.
As for insulation on sleeping pads and in tents, it would take three to five feet of rubber to insulate you from that current outside an urban area so no, your sleeping pad and shoes aren't going to do shit for you. Getting away from the trees, while the reasoning makes sense (yes, they are most likely to get hit and the rapid boiling of their sap causing them to explode would not be a good time for you), it isn't a smart thing to move away from them. When you get as far from the trees as you can, you become the tallest thing in the area you have just moved to, so you're now the target in that spot.
For getting to a low point, not a good choice either- the electricity is trying to get to ground...you just made yourself ground. That goes for caves, rock faces and "shelves" through which the electricity will pass en route to ground too (I always cringe when I see rock climbers getting ready to head up to the rock at 2:30 in the afternoon, thunderhead building).
The reality is, if you're in the wilderness in a lightning storm you're in a bad place. In Denver people get hit in parking lots and at t-ball games all the time- seriously. Look it up. In the wilderness, you're not only outside an area where concrete and asphalt provide a significant resistor to impede the current and buildings have lightning rods on them to attract lightning to them, but you are in a situation where you better know Wilderness First Aid (16 hour class) and what to do when (not if) someone gets hit.
Sit on your butt at the base of a average height tree, in an area of average elevation ground for that area, grab a snack and a book, put on your rain gear, and be 100' from any gear or other people as you can (people generate small electric fields which compound with proximity and think camp stoves, knives, all the little metal pieces of gear you have in your pack...for that matter, make sure your phone and pocket knives are in your pack or away from you too) and settle in until there is a 40 one-thousand count between lightning and thunder. No option is a good one, but this is the most practical. Does it work? Who knows. Is there enough documented case study to provide a scientifically backed answer to the question on any choice? Nope. This one just makes the most practical sense given what we know about lightning's behavior and our understanding of the effects of electricity on the human body.
As for hammocks- no clue and everybody is guessing. There is no information on how far lightning will travel up a tree to bridge to another (hammock attachments tend to be 4-6 feet up a tree trunk depending on distance, tree, user, etc). Most backpacking hammocks are made of parachute material which would most likely melt before the electricity could transfer through me. Could it arc/jump up from the ground to me while I'm in my hammock? Maybe, but then where is it going to go? Remember, it wants to find ground and it would have hit me at a low point. More likely to give me a zap (like a static shock when you touch a metal door knob after walking around carpet in your socks) and melt a hole in my hammock than travel from my butt, up my body, up the line to the tree, and back down to the ground. I'm no electrician, but that doesn't make sense to me from the electricity portion of High School Physics class (it was public school, so I guess you never know).
The biggest potential danger is that it travels up the tree, down the hammock line, through me, out my hip and arcs to the ground. That only works if it goes up my torso though. It is very unlikely that, if it did go up the tree and chose the path down the dry nylon hammock line (I use a rain fly which keeps the line- thus me- dry) rather than back down the most direct path of the already conductive tree that it would then, ratehr than taking most direct path to the ground from my ass arcing down, instead go back up the other side and down the other tree. That only works if the hammock is running directly parallel to the ground- mine sags down quite a bit when I get in it, kind of like...a hammock.
Again, there is no documented information on this- there are no cases yet and even one or two anecdotal cases would tell us very little. It is all speculation based upon our understanding of what we believe lightning behavior to be. Just as we thought tourniquets would cause you to lose the limb if you applied one on based upon our understanding of human anatomy and physiology but no real data to drive it and have now learned that it just isn't the case and have manufactured tourniquets for use at the Basic First Aid level, all of this is based upon our understanding of what we believe lightning behavior to be and it's interaction with the natural world in wilderness settings but we may someday have actual data driven guidelines which prove or disprove some or all of it. We do, however, have enough data to know that was an offensively long sentence for the grammar police (see what I did there?).
The end answer is: it sucks to be in a lightning storm anywhere, in the wilderness especially. Limit the contact you have with the ground to ensure there isn't a direct path between points above and below the heart, ensure that you aren't the tallest thing around (or next to it), don't be close to other people or gear and make sure you aren't on the highest or lowest ground or on a shelf, cave or wall the lighting will travel down on its path to ground. Other than that, take a Wilderness First Aid course before you spend time in the wilderness so you know what to do when someone eventually does get hit.
To answer the next question- yes. You do need to get out of your tent at night. On more than one occasion I have been up at three in the morning against a tree being pissed on my Mother Nature while contemplating whether I really cared if I got hit or not enough to be out in that shit.
I'm getting ready to guide a climb next week and, as for me, I'll choose to remain in the hammock if the weather comes in. I think it will be the safest bet (and, really, it is all just a bet).
Surely the Hammock. Since you're off the ground, and your String and the trees are no conductors, you won't get hit by lightning, since you're not grounded.
In a tent the lighning will burn through your tent and melt the material of your tent. Also since your tent touches the ground and most likely has lots of rainwater on it, it will attract the ligning.
But if you are in a forest, the lightning would hit a tree before you anyways.
My suggestion is to look for a spot with not so tall trees. This way it's more likely, the lightning will hit somewhere else, and if it hits close to you and a tree falls, it's also more likely the tree won't hit you. Maybe look for a dense forest, so the trees and branches can catch the falling tree and it wont fall all the way to the ground.
If you are on a plainland or a area with no trees, maybe think about not building up anything at all. Just wrap your gear and yourself in the tarp to make it mostly waterproof. look for a area close to something tall, but not so close that if it falls it will hit you.
Hope this helps a little bit. Good luck and come home Safe!