so true, still, depends on what kind of material you toke on, before i moved to sweden i had enough place to grow my own and i was smoking about 7 grams a day (it is incredibly much but it actually got the opposite effect it has on regular smokers, but everybody reacts a different way, i havn't been able to focus at school at all before i started to dare to go to school high, it was only then i was really understanding what i was learning and most of all, that i was remembering my lessons more than 15 minutes after the courses, so i actually kept failing my exams and graduation untill i passed all the tests high).
However a lot of materials are way too heavy still not to fall asleep too deep, Haschish and especially black haschishs are to avoid because they really make you overtired, and pure sativa weeds are also to avoid because they have a tendency to lead to a headache when the stoning grows down. i advise pure indica plants, especially the Black Domina from the sensi seeds bank, the blast is really huge and violent (it is probably the strongest weed i have ever tried, and i have tried many this is why i finally choosed to only grow this kind, but on the opposite of other weeds or hasch, it doesn't make you really tired, more contemplative, this is why i was able to succefully cumulate my studies at the sorbonne and a job in conservation of historical documents, waking up 5am and not getting home before 20pm everyday), you must really choose well the grass you fly on, one that increase your ability to focus and concentrate instead of just making you sleepy, and those weeds are not really the majority of them, and you also must know your reactions and limits to canabis, then it just gets a bit easier to access lucid dreaming.
once again everybody has his own methods and little tricks, don't try it if you are a bit freaked or if you don't know how you respond.