Soon after Macbeth murders Duncan, he reports to lady Macbeth: "Methinks I heard a voice cry, Macbeth has murdered sleep . . . " Thereafter he goes on to list the benefits of restful sleep. Sleep thus becomes symbolic of the rewards of a clear conscience as a restful sleep rejuvenates one. But as Macbeth says, by murdering Duncan, he will no longer be able to sleep restfully. Instead, he will become an insomniac. Insomnia over a period of time, can lead to all kinds of mental illnesses. Lady Macbeth too is unable to have restful sleep - and hence in the sleepwalking scene, the audience sees her walking around trying to get rid of her guilt by washing her hands repeatedly. Thus at one end of the spectrum is restful sleep and sanity; at the other end are insomnia, nightmares and instances of sleepwalking.