Amyloid-beta (A beta) accumulation in the brain extracellular space is a hallmark of Alzheimer's disease. The factors regulating this process are only partly understood. A beta aggregation is a concentration-dependent process that is likely responsive to changes in brain interstitial fluid (ISF) levels of A beta. Using in vivo microdialysis in mice, we found that the amount of ISF A beta correlated with wakefulness. The amount of ISF A beta also significantly increased during acute sleep deprivation and during orexin infusion, but decreased with infusion of a dual orexin receptor antagonist. Chronic sleep restriction significantly increased, and a dual orexin receptor antagonist decreased, A beta plaque formation in amyloid precursor protein transgenic mice. Thus, the sleep-wake cycle and orexin may play a role in the pathogenesis of Alzheimer's disease.