5. Critical need for the study of individual differences in sleep-reward circuitry
There is significant overlap between the neural circuitry controlling the motivation for reward and the circuitry controlling sleep-wake states (Fig. 1). Given the many points of interaction between these systems, it is easy to see why these behavioral states are so closely intertwined, and how acute or chronic sleep disturbances can cause such profound changes in the consumption of food, drugs, and alcohol. Individual differences are a major concern for the study of both addiction and sleep disorders. There is tremendous individual variation in how these conditions develop, the effects they have on health and social functioning, and how they respond to treatment. Rodents show natural phenotypic differences in the degree to which food-paired cues engage mesolimbic dopaminergic activity and elicit incentive motivational states. These individual differences have a strong genetic component, and are associated with other behavioral traits that are frequently comorbid with addictive tendencies. By taking advantage of these individual differences, it may be possible to determine whether certain sleep patterns represent an underlying predisposing factor for addictive behavior. For example, a predisposition for poor quality or fragmented sleep, prior to any drug or reward exposure, may be one of several traits that are part of an addictive phenotype. Disordered sleep may cause certain individuals to experience greater attraction to incentive cues when they are first encountered (leading to sign-tracking), which may then be exacerbated by further cue exposure or by the sleep deficits that can result from the comsumption of drugs or alcohol. Another possibility is that baseline sleep characteristics do not differ between STs and GTs, but the hyper-responsive mesolimbic circuitry that underlies sign-tracking may render STs especially vulnerable to the negative effects of sleep deprivation on reward-seeking behavior. In either case, the ST/GT model could provide a better understanding of how the neural pathways mediating sleep and motivation interact with each other, and ultimately lead to treatment strategies for substance use disorders that are more closely tailored to the unique needs of each invidual.