Before the witches prophesize to Macbeth they vow to whip up a storm and destroy the ship of a sailor. Interestingly the witches do not say that they want to murder the sailor. For the witches the inability to sleep is symbolic of a tormented soul. The man who can not sleep lives in chaos, night is day and day is night.
Characters invoke the word sleep as a symbol of order.
But in the play sleep is also a complicated term because it represents a characters control over there lives. When characters can't control their sleeping habits they have entered into the realm of chaos where the fire burn and cauldron bubble. Macbeth his arms soaked in blood after murdering Duncan turns to Lady Macbeth. Macbeth's first admonition that his decision to murder Duncan has destroyed him, is his recognition that he will no longer be able to sleep. Racked by guilt Macbeth instantly recognizes that the order around him is turned upside down.