Anybody who wants to use social drugs should take a look at meditation and self-hypnosis, because you can get the same emotional states that you'd get with drugs through a little practice. As lionhearted mentioned, hallucinogens are the one exception.
I'd like to add the following note: for me, at least, while regular marijuana use did not particularly affect my academic abilities, it did affect my ability to meditate, both while under the influence and during the following day.
"Proper" meditation (where you forget yourself and bask in theta waves) is a definite state which one can reliably enter with regular practice. As a long-time meditator before ever trying pot, I was used to having this resource 15 minutes away, whenever I needed it.
I discovered that it was nearly impossible to achieve this state under the influence or the following day. I enjoyed using pot for it's own sake - but, I would urge you to consider your priorities in mental development. I eventually quit pot because the meditative state was so much more essential.
After training myself to remember at least one dream whenever I sleep, a few hours of REM is more than enough hallucinating each day. Some might say it is better to be hallucinating while running about the real world, but I prefer the ability to instantly wake up out of a 'bad trip'.
The other way around. Sleep deprivation and fevers can indeed induce hallucinations, but I would speculate that they are not as pleasant or controllable as hallucinations from drugs.
Anybody who wants to use social drugs should take a look at meditation and self-hypnosis, because you can get the same emotional states that you'd get with drugs through a little practice. As lionhearted mentioned, hallucinogens are the one exception.