Sleep is a key medium through which foreshadowing occurs in Dracula. Any character who has bad dreams or trouble sleeping is marked for future affliction, as Lucy is in Chapter 6:
Lucy has not walked much in her sleep the last week, but there is an odd concentration about her which I do not understand; even in her sleep she seems to be watching me. She tries the door, and finding it locked, goes about the room searching for the key.
Lucy's trouble sleeping foreshadows Dracula's interference and attack, as well as Lucy's future death and mutation into a vampire. As Lucy's sleeping troubles worsen, so does her death harken nearer, as Dracula preys on her at night. This reappears as a motif in Dracula, with both Mina and Jonathan having bad dreams and/or trouble sleeping as a prelude to danger.
As a means of foreshadowing tragedy, sleep is a particularly terrifying medium. Human beings are in their weakest, most vulnerable state while sleeping, readily available for exploitation. As a means of generating tension and contributing to the general mood of foreboding and horror, Stoker could not have made a better creative choice. In removing sleep and nighttime as sources of respite, Stoker ensures that both his characters and his readers remain on edge.
Towards the end of Chapter 6, Stoker uses a combination of foreshadowing, imagery, and simile to generate a sense of dread and foreboding:
Today is a grey day, and the sun as I write is hidden in thick clouds, high over Kettleness. Everything is grey - except the green grass, which seems like an emerald amongst it . . . . Dark figures are on the beach here and there, sometimes half shrouded in mist, and seem "men like trees walking."
Mina's description of the physical landscape of Kettleness - its menacing ambience - creates a mysterious and unsettling image. Like the "dark figures" obscured by mist, the dark events of the future are obscured from both Mina and the reader, though the ominous mood created by this natural imagery foreshadows some series of menacing supernatural events.
Furthermore, the simile/image of "men like trees walking" reinforces the fact that supernatural forces are at work, conjuring up the image of enchanted trees walking up and down the beach. The statement "men like trees walking" is also an allusion to a Biblical passage in the Gospel of Mark, wherein Jesus heals a blind man. This biblical allusion contributes to the foreshadowing in this passage, implying that something that's unclear will soon be revealed.
