Deja vu is an uncanny feeling or illusion of having already seen or experienced something that is being experienced.It can be a illusion or delusion.and dreams are related to our sub conscious mind. sub conscious looks at what happens the whole day in his own way and then sees it in dreams, or your hidden fears or feelings might come as dreams.Dreams are very complex process and are not fully understood by scientists. but there r drugs that can give u good dreams and also which can give u night mares.