How the brains of people who sleep during the day differ
The authors of a new study found a causal relationship between sleep and brain volume.The new study found that those people who are genetically predisposed to get regular daytime naps have larger and healthier brains.
Previous research has shown that sleep can improve cognitive performance, with short naps (5 to 15 minutes) providing a one- to three-hour advantage. Aging reduces reaction time and memory and often leads to an increased prevalence of cognitive impairment. With the world's aging population, it is important to identify modifiable risk factors associated with cognitive performance, particularly sleep habits. To this end, researchers from University College London conducted a study examining whether there was a causal relationship between daytime sleep and brain health.
The study involved 378,932 people of European descent between the ages of 40 and 69. Using the Mendelian randomization technique, they studied DNA fragments to determine the likelihood of people sleeping habitually. Mendelian randomization is a method of using measured gene variations of a known function to study the causal effects of variable risk factors.
All participants were given cognitive tests of visual memory and reaction time. Researchers also reviewed MRI scans of some participants' brains for structural brain changes. In addition, the subjects were asked to talk about their sleeping habits.
The researchers compared the brain health and cognitive functioning of people who were genetically "programmed" to sleep with those who were not in the habit of sleeping during the day. They found that, in general, people who are programmed to sleep have greater overall brain volume, a marker of brain health, especially in older people. Decreased brain volume, also known as atrophy, is associated with diseases such as cognitive impairment and dementia.
The average difference in total brain volume between the two groups of people was 2.6 to 6.5 years of aging. But there was no difference in other measures -- hippocampal volume, reaction time and visual processing. The hippocampus is a complex structure deep in the brain that is associated with memory and learning. Hippocampal volume, in particular, is associated with cognitive decline.
Based on these findings, the researchers said there is a "modest causal relationship" between habitual daytime sleep and greater total brain volume.
The study is published in the journal Sleep Health.