Alzheimer's disease is associated with sleep disorders but the precise relationship is unclear.[165][166] It was once thought that as people get older, the risk of developing sleep disorders and AD independently increase, but research is examining whether sleep disorders may increase the prevalence of AD.[165] One theory is that the mechanisms to increase clearance of toxic substances, including Aβ, are active during sleep.[165][167] With decreased sleep, a person is increasing Aβ production and decreasing Aβ clearance, resulting in Aβ accumulation.[168][165][166] Receiving adequate sleep (approximately 7–8 hours) every night has become a potential lifestyle intervention to prevent the development of AD.[153]