A 6.5-hour nap preserved the cerebral cortex
An analysis of magnetic resonance imaging (MRI) scans of nearly four thousand people has provided no evidence of a link between sleep duration and brain matter atrophy. In contrast, scientists from seven countries found that 6.5 hours of sleep was associated with the greatest thickness of the cerebral cortex of the large cerebral hemispheres. The study is published in Nature Human Behavior.
Most guidelines suggest that adults should get at least seven to eight hours of sleep each night. However, it is widely believed that shorter sleep duration may have negative effects on health: in particular, it may increase the risk of developing Alzheimer's disease. However, it is still unknown exactly what length of sleep has a favorable effect on brain health, and whether there is a causal relationship between sleep duration and brain health - the development of atrophy of brain matter.
Anders Fjell (Anders Fjell) from the University of Oslo with colleagues from the UK, Denmark, Germany, Spain, Switzerland, and the United States studied the relationship between sleep duration and the incidence of brain atrophy. To do this, they used data from 8153 longitudinal MRI brain scans from 3893 participants (20-89 years old). They also conducted additional analyses using 51295 MRI scans from 47029 participants.
Respondents' average sleep duration was relatively stable at about seven hours across the lifespan, although it was significantly associated with age (p < 2 × 10-16). Analysis of tomograms showed that longer sleep was linearly associated with greater loss of posterior brain matter volume, and shorter sleep was associated with greater loss of cerebellar and thalamic white matter volume. When adjusted for covariates, the decrease in substance volume in posterior regions was no longer statistically significant. However, a more detailed statistical analysis showed that damage to most brain structures showed a significant inverse U-shaped dependence on sleep duration.
It appeared that 6.5 hours of sleep was associated with the maximum volume of subcortical structures, the smallest volume of brain ventricles, and the thickest cortex of the large hemispheres. The confidence interval in which similar effects were observed ranged from 5.7 and 7.3 hours. Adjustments for body mass index and symptoms of depression had no detectable effect on the results.
A full-genome association search showed that genes associated with longer sleep in those with less than average sleep were as active as genes for shorter sleep in those with longer than average sleep. The scientists hypothesized that there is a genetically determined desire for average sleep duration. They also found that genes involved in lipid metabolism may be responsible for inter-individual differences in sleep duration in those who sleep less than average. In addition, they found that genes associated with larger brain volumes were associated with longer sleep in people with short sleep duration.
Thus, scientists have established a new optimal sleep duration sufficient for brain preservation - 6.5 hours. At the same time, extreme deviations from this duration can lead to disorders of the brain structure. In the future, additional studies will be needed to understand the impact of sleep quality on these indicators.