brachot 55

until the letter is read, don't know if the dream is from the angels or the demons.

but not everything that stands in the shadows is something to fear - depending on your heart, they could bring wisdom and insight.

(apologies to Goya. for english translation of this talmud page see here)

- it says on this page that it is better to have a bad dream, as that can inspire reflection and remorse. or is a way of receiving punishment in this world. so this yom kippur, fast well and wish you not-such sweet dreams