What’s the Magic Number for Sleep?

Consistently sleeping 7 hours per night was associated with optimal cognitive function and mental health for middle-aged adults, a new study found.

Sleep disturbances are common in older age, and previous studies have shown associations between too much or too little sleep and increased risk of cognitive decline, but the ideal amount of sleep for preserving mental health has not been well described, according to the authors of the new paper.

In the study published in Nature Aging, the team of researchers from China and the United Kingdom reviewed data from the UK Biobank, a national database of individuals in the United Kingdom that includes cognitive assessments, mental health questionnaires, and brain imaging data, as well as genetic information.

Sleep is important for physical and psychological health, and also serves a neuroprotective function by clearing waste products from the brain, lead author Yuzhu Li of Fudan University, Shanghai, China, and colleagues wrote.

The study population included 498,277 participants, aged 38-73 years, who completed touchscreen questionnaires about sleep duration between 2006 and 2010. The average age at baseline was 56.5 years, 54% were female, and the mean sleep duration was 7.15 hours.

The researchers also reviewed brain imaging data and genetic data from 39,692 participants in 2014 to examine the relationships between sleep duration and brain structure and between sleep duration and genetic risk. In addition, 156,884 participants completed an online follow-up mental health questionnaire in 2016-2017 to assess the longitudinal impact of sleep on mental health.

Both excessive and insufficient sleep was associated with impaired cognitive performance, evidenced by the U-shaped curve found by the researchers in their data analysis, which used quadratic associations.

Specific cognitive functions including pair matching, trail making, prospective memory, and reaction time were significantly impaired with too much or too little sleep, the researchers said. “This demonstrated the positive association of both insufficient and excessive sleep duration with inferior performance on cognitive tasks.”

When the researchers analyzed the association between sleep duration and mental health, sleep duration also showed a U-shaped association with symptoms of anxiety, depression, mental distress, mania, and self-harm, while well-being showed an inverted U-shape. All associations between sleep duration and mental health were statistically significant after controlling for confounding variables (P < .001).

On further analysis (using two-line tests), the researchers determined that consistent sleep duration of approximately 7 hours per night was optimal for cognitive performance and for good mental health.

The researchers also used neuroimaging data to examine the relationship between sleep duration and brain structure. Overall, greater changes were seen in the regions of the brain involved in cognitive processing and memory.

“The most significant cortical volumes nonlinearly associated with sleep duration included the precentral cortex, the superior frontal gyrus, the lateral orbitofrontal cortex, the pars orbitalis, the frontal pole, and the middle temporal cortex,” the researchers wrote (P < .05 for all).

The association between sleep duration and cognitive function diminished among individuals older than 65 years, compared with those aged approximately 40 years, which suggests that optimal sleep duration may be more beneficial in middle age, the researchers noted. However, no similar impact of age was seen for mental health. For brain structure, the nonlinear relationship between sleep duration and cortical volumes was greatest in those aged 44-59 years, and gradually flattened with older age.

Research Supports Sleep Discussions With Patients

“Primary care physicians can use this study in their discussions with middle-aged and older patients to recommend optimal sleep duration and measures to achieve this sleep target,” Noel Deep, MD, a general internist in group practice in Antigo, Wisc., who was not involved in the study, said in an interview.

“This study is important because it demonstrated that both inadequate and excessive sleep patterns were associated with cognitive and mental health changes,” said Deep. “It supported previous observations of cognitive decline and mental health disorders being linked to disturbed sleep. But this study was unique because it provides data supporting an optimal sleep duration of 7 hours and the ill effects of both insufficient and excessive sleep duration.

“The usual thought process has been to assume that older individuals may not require as much sleep as the younger individuals, but this study supports an optimal time duration of sleep of 7 hours that benefits the older individuals. It was also interesting to note the mental health effects caused by the inadequate and excessive sleep durations,” he added.

As for additional research, “I would like to look into the quality of the sleep, in addition to the duration of sleep,” said Deep. For example, whether the excessive sleep was caused by poor quality sleep or fragmented sleep leading to the structural and subsequent cognitive decline.

Study Limitations

“The current study relied on self-reporting of the sleep duration and was not observed and recorded data,” Deep noted. “It would also be beneficial to not only rely on healthy volunteers reporting the sleep duration, but also obtain sleep data from individuals with known brain disorders.”

The study findings were limited by several other factors, including the use of total sleep duration only, without other measures of sleep hygiene, the researchers noted. More research is needed to investigate the mechanisms driving the association between too much and not enough sleep and poor mental health and cognitive function.

The study was supported by the National Key R&D Program of China, the Shanghai Municipal Science and Technology Major Project, the Shanghai Center for Brain Science and Brain-Inspired Technology, the 111 Project, the National Natural Sciences Foundation of China and the Shanghai Rising Star Program.

The researchers had no financial conflicts to disclose. Deep had no financial conflicts to disclose, but serves on the editorial advisory board of Internal Medicine News.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

Source: Read Full Article