Harvard Medical School studied more than eights hours sound sleep   is important to sleep less than 25 Degree celcius Temprature .

2,800 individuals ages 65 and older participating in the National Health and Aging Trends Study to examine the relationship between their self-report of sleep characteristics in 2013 or 2014, and their development of dementia and/or death five years later. Researchers found that individuals who slept fewer than five hours per night were twice as likely to develop dementia, and twice as likely to die, compared to those who slept six to eight hours per night. This study controlled for demographic characteristics including age, marital status, race, education, health conditions, and body weight.