It goes without saying that sleep is important. Some people need more than others and the rare person can perform well mentally on low amounts of sleep routinely. (I envy those people!)
Recently, two articles looked at sleep and cognition. The first was called Sleep and cognitive decline: A prospective nondemented elderly cohort study, and looked at 2893 Korean adults older than 60 years. Around 2200 were considered cognitively normal and the rest had Mild Cognitive Impairment (MCI.) MCI means that there is cognitive loss considered to be more than normal for age, but not disabling. When cognitive decline is disabling, that’s when we call it dementia. The subjects in the study filled out questionnaires about sleep at the beginning of the study and were reassessed 4 years later. The study found that longer sleep latency (taking more than 30 minutes to fall asleep) and longer sleep hours (more than 7.95 hours) were both associated with increased risk for cognitive decline in the initially normal subjects. There was 40% and 70% increased risk, respectively. Surprisingly, a later midpoint of sleep (after 3AM) was protective and associated with a 40% reduced chance of developing cognitive decline. (So much for early to bed early to rise!) But remember these are just associations. It’s not proven that certain sleep habits increase dementia.
While the above study looked at general sleep habits, another recent study looked at sleep deprivation and levels of toxic β-amyloid, the substance thought to cause Alzheimer’s disease. The study, called β-Amyloid accumulation in the human brain after one night of sleep deprivation, took 20 healthy volunteers and performed a PET scan that images levels of β-amyloid after a night of normal sleep and a night of complete sleep deprivation. They found that amyloid was higher in key parts of the brain after sleep deprivation. This study doesn’t prove at all that sleep deprivation causes Alzheimer’s disease but it is very interesting that a potentially problematic brain waste product is cleared out with sleep and not cleared out without sleep.
I wouldn’t lose sleep over any of this! If you are an early sleeper and early riser that’s just you! If you need 9 hours of sleep to feel normal, that’s ok too! But listen to your body and brain and get the sleep that you feel you need, when you need it. And while you’re awake use the time to get your brain plenty of stimulating activity along with physical exercise.