It is a well-known fact that sleep is important. It’s important for regeneration of our tissues in every single organ in our body, including our brain. A decline in our mental capacity is the most immediate effect we see when we get poor quality of sleep. If we accumulate all those less-than-six-hours-of-sleep-and-I’m-fine nights, it may actually lead to many chronic diseases—two of them being dementia and Alzheimer’s disease. Our time in bed (reflected by the time we go to bed and sleep, the length of sleep, and the time we wake and get up) may actually be factors for cognitive decline.

This week, I explore the association of time in bed with cognitive decline & risk for dementia and Alzheimer’s disease as we age.

A study investigated a Chinese population located in the rural areas of China. This is significant because about 5% of China’s population is affected by dementia—the largest percentage in the world. In the study, the authors used subject-reported information regarding their sleep habits (the main limitation of the study). Aspects of sleep habits included the time subjects go to bed, time subjects wake up, excessive daytime sleepiness, incidents of sleep fragmentation, and sleep duration. Sleep quality was assessed by tools such as the Pittsburg Sleep Quality Index. A group of neurologists then examined each subject for incidence of dementia, Alzheimer’s disease, and/or signs of cognitive decline. It’s important to note that the mean age of the subjects was 70 years old.


The authors of this study concluded that:

•    Short and long sleep duration*, time in bed, and early* and late* sleeping time were associated with risk of dementia, as reflected by disrupted circadian rhythm.
•    Persistent long and prolonged time in bed showed greater risk of cognitive decline.

Sweet spot sleep duration: 

7-8 hours

•    Short – less than that
•    Long – more than that

Early: 9 PM
Late: 10 PM-1:30 AM


According to the results of this study, the bottom line is that sleep duration that is too short and too long puts an individual at risk for cognitive decline, dementia, or Alzheimer’s disease. The best length of time of sleep is 7-8 hours per night. Lastly, the best time to go to bed seems to be 10 o’clock at night.

This study has many limitations, and it has many aspects where future research can delve in further. The new information here is that there seems to be “the best time to go to sleep” and length of sleep may not be the only factor to sleep quality. This, unfortunately, has definite implications on people that work night shifts.


Disclaimer: Information from this blog is not intended to be as a resource used for self-help or self-diagnosis. The information posted should be regarded as general information based on what Isabel Sison deems as reliable. She does not endorse this information as a substitute to formal medical consultation and treatment. Any change in the information is possible without prior notice.