(Reposted)
A couple of months ago, I talked about Hamlet's major soliloquy in Act III, scene i of Hamlet, focusing on the originality of his identification between sleep and death. In my Bardolatrous zeal, I may have overstated this somewhat. Make no mistake; Hamlet does expand upon and develop this idea to near perfection. But, there were hints to this connection long before even the historic Hamlet was born in Medieval Denmark.
The idea that sleep and death could be at least metaphorically connected is first hinted at in Greek mythology (pretty much everything in modern culture is ultimately derived from either Shakespeare or the Greeks and Romans). Specifically, the daemons (supernatural beings higher than humans, but not gods) who represented Sleep (Hypnos) and Death (Thanatos) were twin brothers, children of Night (Nyx) and Primordial Darkness (Erebus). Whilst they don't directly interact with each other in any surviving myths, there is at least one image (an illustration of a scene from Homer's Iliad on a vase) where, significantly, they're working together to carry a fallen hero (Sarpedon) to his final resting place.
Meanwhile, in Roman literature, we have another tantalizing hint at this connection in Virgil's epic, The Aeneid. In Book VI, the Trojan prince and legendary founder of Rome, Aeneas, must venture into the Underworld so that he can get directions from his recently deceased father on where to go next in his journey. When this is done, Aeneas must leave the Underworld by exiting through an odd gate, with one half made from ivory, and the other half out of horn. This is interesting because, in Classical mythology, dreams were said to come to the mortal world through a gate made from horn and ivory. This has lead to several interesting theories that Virgil may be inferring a connection not only between the states of sleep and death, but he also may be hinting that the afterlife is a sort of psychic state.
In an example closer to Shakespeare's era, this idea was also explored by the 14th century Florentine poet, Dante Alighieri (another major source of Everything), in the Inferno section of The Divine Comedy. One of Dante's major innovations was to make the torments of Hell, a region he explores with Virgil, the physical manifestation of the psychic state during the sin being punished. This implies that Hell has a deep connection with our mental states, but Dante, a devout Medieval Catholic, still maintains the objective existence of Hell. Given how much Italian culture influenced Shakespeare's England (several plays of the era, including many of Shakespeare's, take place in Italy), it's very likely that Shakespeare was influenced by Dante. And finally, in Doctor Faustus, Mephistopheles says that Hell is anywhere without God. And the writer of Doctor Faustus was Christopher Marlowe, a contemporary of and major influence on Shakespeare.
Greek myth, Roman literature, and Dante- with these rivers of thought flowing into his mind as raw material, it should be no surprise that Shakespeare was able to create the masterpieces that he did. However, it takes more than just having the right inspirations to create literature on the level that he did- it also requires natural genius.