It has now become clear that the relation of sleep and pain is bidirectional; acute and chronic pain is associated with disturbed sleep and disturbed sleep enhances pain. Experimental studies have shown that reduced and fragmented sleep in pain-free normals increases their pain sensitivity and daily self-report studies in chronic pain patients have shown a poor night of sleep is followed by enhanced next-day pain. In mediation analyses of large clinical data sets it is found that the sleep-pain side of the bidirectional relation, as opposed to the pain-sleep side, accounts for the greater variance. These data then would suggest that improving sleep in chronic pain disorders should attenuate daytime pain.