Lucky Number 7: Seven Hours Is the Ideal Amount of Sleep for People in Middle Age and Upwards

Whether you’re falling asleep at the wheel from lack of sleep or stuck in a prolonged slumber in bed, poor sleep is terrible for health. Dozing off for too long can affect brain health, and too little shut-eye can have us sleeping with the fishes a bit earlier. So, how many sheep should we be counting every night to keep our brains healthy and our bodies alive and kicking?  

Using data from the UK Biobank for participants primarily of European ancestry aged 38 to 73 years, including 94% white people, researchers from the UK and China identified approximately seven  hours as the optimal sleep duration. Both insufficient and excessive sleep duration were significantly associated with a decline in cognition. These findings have emphasized the importance of sleep regulation for adults' cognition, mental health, and well-being.

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Alterations in sleep patterns — including difficulty falling asleep, staying asleep, and decreased quantity, quality, and efficiency of sleep — are important characteristics of the aging process. So, sleep disturbances are prevalent in the aging population and may be accompanied by cognitive decline and poorer well-being. A recent study showed an inverted U-shaped association between sleep duration and global cognitive decline. Sleep duration of fewer than four hours or more than ten hours is detrimental.

Abnormal sleep is associated with detrimental changes in brain structures in older populations. Shorter total sleep duration in middle-aged and older adults has been linked to impairment in white matter organization – the highways of cell connections in the brain. There’s evidence that age-related atrophy of the brain regions involved in sleep regulation may contribute to the emergence of sleep irregularity in the aging population.

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Published in Nature Aging, researchers from the University of Cambridge and Fudan University showed links between sleep duration, mental health, cognitive function, and brain structure in a large cohort of middle-aged to older participants. The study showed a beneficial association with cognitive function and mental health with a sleep duration of approximately seven hours in middle-aged to older adult populations.

This association between sleep duration and cognitive performance potentially suggests that insufficient or excessive sleep duration may be a risk factor for cognitive decline in aging. A possible reason for the association between insufficient sleep duration and cognitive decline may be the disruption of slow-wave sleep, which has been identified as having a close association with memory consolidation. A reduction in sleep time may have detrimental consequences on the clearance of toxins. It is possible that prolonged sleep duration results from poor quality and fragmented sleep.

The most significant brain structures affected in this study included regions tied to decision-making (the precentral cortex and the lateral orbitofrontal cortex) and memory (the hippocampus). Non-optimal sleep duration was significantly associated with decreased cognitive function and increased indicators of poor brain health. Furthermore, age-related atrophy of brain regions involved in regulating sleep and wakefulness may contribute to circadian dysfunction and decreased production and secretion of melatonin in older adults.

The study demonstrates that the association between sleep duration,cognitive function, and mental health gradually diminishes in a population aged above 65 years compared with a middle-aged population of 40 years in age. This distinction might be explained by the gradual increase in sleep disturbances with age and the frequent occurrence of fragmented sleep, which may contribute to the attenuated nonlinear association between optimal sleep duration and mental and cognitive health in a population aged over 66 years. The results demonstrate that optimal sleep duration may be more beneficial to the middle-aged population, likely related to their engagement in occupational activities and skills.

Professor Barbara Sahakian from the Department of Psychiatry at the University of Cambridge, one of the study's authors, said, "Getting a good night's sleep is important at all stages of life, but particularly as we age. Finding ways to improve sleep for older people could be crucial to helping them maintain good mental health and wellbeing and avoiding cognitive decline."

References:

Li, Y., Sahakian, B.J., Kang, J. et al. The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition and mental health. Nature Aging 2, 425–437 (2022). https://doi.org/10.1038/s43587-022-00210-2