Con: Alzheimer's disease and circadian dysfunction: Chicken or egg?

The development of late-onset Alzheimer's disease is believed to be influenced by genetic, socioeconomic, and lifestyle factors. Recently, converging research in animal and human studies has found that beta-amyloid (A) levels in cerebrospinal fluid are modulated by sleep-wake cycles. This raises the possibility that chronic sleep loss causes brain amyloid accumulation over time and leads to the development of Alzheimer's disease. The observation that circadian rhythm modulates A levels has not yet been replicated by other groups, and subject selection and methodologies are potential explanations for this. While acute suppression of sleep may raise A levels, it is not known whether chronic sleep loss has the same effect. It is conceivable that altered circadian rhythms are a manifestation of a disrupted sleep network because of preclinical disease, as has been observed in other neurodegenerative disorders. The findings that circadian variation in A levels in cerebrospinal fluid is a direct result of sleep-wake cycles and that altering normal rhythms increases the risk for brain amyloid accumulation need to be replicated in larger cohorts. Prospective studies are needed to decipher whether circadian rhythm dysfunction is a cause, or a result of, amyloid accumulation.