Dementia risks rise with shorter night sleep: study POOR SLEEPERS: Researchers found a 30% greater risk of dementia in those who during their 50s, 60s and 70s routinely got six hours of sleep or less each night The Guardian People who regularly sleep for six hours or less each night in middle age are more likely to develop dementia than those who routinely manage seven hours, a major study into the disease showed. Researchers found a 30 percent greater risk of dementia in those who during their 50s, 60s and 70s consistently had a short night’s sleep, regardless of other risk factors, such as heart and metabolic conditions, and poor mental health.