Sleep and circadian dysfunction in neurodegenerative disorders: insights from a mouse model of Huntington's disease.

Sleep disorders are common in patients with neurogenerative diseases and manifest early in the disease process. Among a number of possible mechanisms underlying the sleep disturbances, there is evidence that dysfunction in the circadian system is a contributing factor. Focusing on a mouse model of Huntington's disease has enabled us to determine that at the onset of symptoms, spontaneous electrical activity of neurons within the central clock is disrupted even though the molecular clockwork is still functional. These findings suggest that the fundamental deficit contributing to disordered sleep is reduced SCN output. The mechanism underlying this deficit is not yet known, but mitochondrial dysfunction and oxidative stress are likely involved. Disruption of circadian output from the SCN would be expected to have wide ranging impact on the body including SCN regulated brain regions and the heart. In fact, there is a great deal of overlap in the non-motor symptoms experienced by HD patients and the consequences of circadian disruption. This raises the possibility that the disordered sleep and circadian function experienced by HD patients may be an integral part of the disease. Furthermore, we speculate that circadian dysfunction may accelerate the pathology underlying HD. If these hypotheses are correct, we should focus on treating circadian misalignment and sleep disruptions early in disease progression.