To all you time travelers: Ever seen Doctor Who, Harry Potter 3 or Back to the Future?
There are sooooo many problems with time travel. Lets just list a couple.
Say, you want to go back in time to keep someone from dying. Your going to have to be incredibly careful. If your past self finds out that the person in peril that you were saving survived, you would remove the reason for which you went back in time. Your past self would never return to the past and you would... cease to exist. This makes traveling to the past incredibly limited to just a few situations. Incredibly dangerous as it is, it would let you get more sleep, as long as you were careful.
Say you want to see what the future is like. So you go there, check out what happens to you and go home. Now, that future you saw is almost sure to change, especially your future, because you now have knowledge of the future and your decisions are sure to change.
There are so many more I could list, like the grandfather paradox, but I won't. I don't have time.
Other variants would work, though, like time freezing and traveling to the past and inhabiting your past body with knowledge of the future came from. (Though that wouldn't work for the future, though, and would be more limited.) Those would work. Lets assume most of you are talking about those.
Yes. I'm a total sci-fi nerd.