Seven to Eight Hours: The Goldilocks of Sleep

The Centers for Disease Control (CDC) suggests that about one third of Americans do not get enough sleep (defined as <7 hours of sleep per night). Poor sleep (both too much and too little) is associated with increased risks for several diseases including cardiovascular disease, diabetes, Alzheimer’s disease, and even mortality. Previous observational studies have suggested that seven to eight hours of sleep is associated with optimal cognitive functions, whereas too much or too little sleep may be associated with long-term cognition impairment [1]. However, most of these studies have been small and did not follow participants for a long period of time.

A recent study [2] combined data from two large observational studies, 9,254 participants from the English Longitudinal Study of Ageing and 10,811 participants from the China Health and Retirement Longitudinal Study. When the participants were first enrolled, they were asked approximately how many hours of sleep they got on a typical weeknight. Over the next four to eight years, the participants’ global cognitive performance, memory, executive function, and orientation (the awareness of one’s self with regards to personal history, time, and place) were evaluated.

At baseline, those who slept less than four hours or more than eight hours had impaired cognition compared to those who slept seven hours on average. Impairment was seen for general cognition, memory, executive function, and orientation. When the investigators followed these individuals over several years, global cognition declined more in those who slept less than four hours or more than ten hours. Together, this study provides strong validation that getting seven or eight hours of sleep is associated with optimal cognitive function.

How might sleep influence long-term cognitive function? First, sleep is important for the consolidation of memories, and poor sleep may increase the risk of memory deficit [3]. In addition, poor sleep may increase the thinning of several brain regions [4] and elevate levels of inflammation [5]. Finally, as we mentioned in other Cognitive Vitality blog posts, poor sleep could increase the levels of misfolded proteins in the brain that are associated with Alzheimer’s disease and may increase the risk of Alzheimer’s disease itself.

However, there are several limitations to the study. It is thought that the association between Alzheimer’s and sleep may be bi-directional, meaning that sleep could increase the risk of Alzheimer’s disease while the earliest manifestations of Alzheimer’s may impair sleep. Also, this was an observational study and cannot confirm whether poor sleep causes cognitive decline or there is simply a correlation. For example, excessive sleep may indicate other underlying conditions, such as depression, that may be associated with cognitive decline itself. Finally, the investigators measured self-reported sleep duration, rather than using an objective measure of sleep.

Getting the right amount of sleep is one of Cognitive Vitality’s “7 Steps for Brain Health.” What can you do to improve your sleep?

• Try to go to bed and get up at the same time every day.
• Try not to eat or exercise within a few hours before bedtime.
• Avoid caffeine and alcohol before bed.
• Do something relaxing before you go to sleep.
• Use your bed for only sleep or sex.
• Try to keep your room dark and comfortable at night.

If all else fails and you are still having trouble falling asleep, consult your healthcare provider. They may be able to identify underlying health conditions that could be impacting your sleep or may provide other treatment recommendations. If sleep problems persist and your doctor suggests medications, be sure to discuss the benefits and risks of sleeping pills, which may themselves impact cognition. Getting a good night’s sleep is an important way to improve overall well-being and brain health.

1. Lo JC, Groeger JA, Cheng GH et al. (2016) Self-reported sleep duration and cognitive performance in older adults: a systematic review and meta-analysis. Sleep Med  17, 87-98.
2. Ma Y, Liang L, Zheng F et al. (2020) Association Between Sleep Duration and Cognitive Decline. JAMA Netw Open  3, e2013573.
3. Dudai Y, Karni A, Born J (2015) The Consolidation and Transformation of Memory. Neuron  88, 20-32.
4. Spira AP, Gonzalez CE, Venkatraman VK et al. (2016) Sleep Duration and Subsequent Cortical Thinning in Cognitively Normal Older Adults. Sleep  39, 1121-1128.
5. Patel SR, Zhu X, Storfer-Isser A et al. (2009) Sleep duration and biomarkers of inflammation. Sleep  32, 200-204.

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