H. P. Lovecraft 
Silver Key 

The Silver Key is a short story written by H.P. Lovecraft in 1926, considered part of his Dreamlands series. Randolph Carter discovers, at the age of 30, that he has gradually "lost the key to the gate of dreams." As he ages, he finds that his daily waking exposure to the more "practical", scientific ideas of man, has eventually eroded his ability to dream as he once did, and has made him regretfully subscribe more and more to the mundane beliefs of everyday, waking "real life". But still not certain which is truer, he sets out to determine whether the waking ideas of man are superior to his dreams. (Goodreads)