New research reveals that age, biological sex, and cognitive ability correspond to distinct patterns in the brain waves that ripple through the mind at night. Analyzing data from nearly two thousand healthy individuals, investigators found that the architecture of rest changes as people get older and differs subtly between men and women. The study was published in the journal Frontiers in Sleep.
When a person drifts off for the night, their brain cycles through various phases. Humans alternate between rapid eye movement sleep, where vivid dreaming typically occurs, and several stages of non-rapid eye movement sleep. The latter includes light rest as well as deep, slow-wave rest.
During these non-dreaming phases, specialized electrical bursts race across the brain. These bursts are known as sleep spindles. Lasting anywhere from half a second to three seconds, these oscillatory waves are generated deep within the brain by a structure called the thalamus and travel outward to the cortex.
Researchers study these electrical events because they are highly associated with memory consolidation and learning. The bursts represent the brain reorganizing itself in response to new information. Diana Campos-Beltrán, a researcher at the University of Lübeck in Germany, and her colleagues wanted to understand how these nighttime patterns vary across the general population.
Many scientists are currently investigating ways to enhance sleep using gentle electrical or auditory stimulation. To make these interventions effective for everyone, researchers need an accurate baseline of how normal rest varies across different demographics. Without knowing how the living brain changes naturally over a lifespan, medical interventions remain difficult to optimize.
To build this comprehensive picture, the research team conducted a meta-analysis. This statistical technique involves pooling the results of many previously published papers to identify trend lines across a vast population. The approach helps smooth out the quirks of individual datasets and often reveals patterns that might not be statistically significant in smaller samples.
The investigators combined data from 42 separate studies into a central database. The final dataset included 1,878 healthy subjects. Each included study had to feature quantitative measurements of sleep spindles as well as complete polysomnography data.
Polysomnography is a comprehensive recording of the biological changes that occur during sleep. The original researchers collected this data using sensors placed on the scalp to measure electrical brain activity in a process called electroencephalography. They also incorporated measurements of eye movements and muscle tension to accurately separate the night into specific sleep stages.
The passage of time relates to distinct shifts in how the human brain rests. The researchers noted that older subjects experienced a decrease in the density of sleep spindles throughout the night. The amplitude, or electrical height, of these brain waves was also diminished in older populations.
The duration of individual sleep spindles dropped as age increased. Beyond these short bursts of electrical activity, the older adults showed reductions in their overall sleep quality. The older participants spent less time in the deepest, most restorative stages of rest.
These individuals also experienced much shorter periods of rapid eye movement sleep. In addition to losing out on deep and dreaming sleep, older study participants spent more time awake after initially falling asleep. Together, these metrics paint a picture of increasingly fragmented and lighter sleep as humans age.
The researchers note that biological changes in the aging brain could explain the reduced sleep spindle activity. As the brain ages, the volume of the thalamus and the integrity of the neural connections to the outer cortex both decline. Reductions in gray matter and white matter could result in less synchronized neuronal firing, muting the electrical signatures picked up by scalp sensors.
Physical changes on the outside of the head could also play a role in the older measurements. The researchers suggest that a thicker skull bone and an increased distance between the shrinking brain and the scalp might reduce electrical conductivity. This would also result in the lower brain wave amplitudes seen in the data from elderly populations.
Biological sex mapped onto differences in sleep architecture as well. Across the collected data, females exhibited higher absolute sleep spindle power than males. This means the overall intensity of this specific brain wave frequency was greater in women, an effect driven largely by the older subjects in the gathered data.
In terms of overall rest quality, sleep was generally more consolidated in females. Males tended to experience more awakenings during the night. The men in the gathered studies also logged less total sleep time than the women.
Females spent a greater duration of the night in deep slow-wave sleep. Overall sleep efficiency, which measures the percentage of time spent asleep while in bed, was lower in male subjects. The team speculates that human biology might underlie the differences in sleep patterns between men and women.
Female sex hormones like progesterone increase the signaling of specific neurotransmitter receptors that help generate sleep spindles and initiate sleep. The hormonal differences between aging men and women might contribute to the divergence in brain wave activity as the two demographic groups grow older.
The research team also looked for connections between nighttime brain activity and waking intelligence. They found that cognitive abilities relate to sleep spindles in ways that depend heavily on age. The combined data showed that higher overall sleep spindle power correlated with higher scores on cognitive tests.
The total count of spindles during the night also showed a positive correlation with intelligence measurements. The relationship became nuanced when the researchers separated fast sleep spindles from slow sleep spindles. Depending on their precise frequency, fast and slow spindles are generated by slightly different regions within the brain.
Humans generate both types of waves, and each type might support different elements of cognitive performance. The cognitive tests used in the original studies varied heavily. Researchers across the 42 studies measured reasoning ability, processing speed, and general intelligence using common psychological testing tools.
For adults and older individuals, a high density of slow sleep spindles was strongly associated with better cognitive test scores. In children, this specific relationship did not appear. Instead, children showed differing patterns, such as a localized association between higher intelligence and a greater density of spindles overall.
The length of individual sleep spindles did not correlate with cognitive ability in any age group. The researchers suggest that spindle duration might have more to do with the specific task a person learned that day rather than their general intelligence. Different learning tasks demand different levels of memory consolidation while we sleep.
Observational studies like this one come with inherent limitations. The researchers caution that differing methods in the original studies could introduce variability into the final pooled results. Measuring and detecting brain waves is not a universally standardized process, and different laboratories use slightly different software thresholds to define a sleep spindle.
Additionally, many previously published studies do not report their non-statistically significant findings. While the researchers contacted authors directly to obtain unpublished data, a lack of available measurements ultimately led to the exclusion of several older papers. The team also could not fully account for physical shifts in where sleep spindles appear on the scalp.
Previous research indicates that the locations of maximum brain wave activity can shift as a person ages. Looking only at standard sensor placements might misrepresent the true power of an electrical event if the activity has shifted to a slightly different part of the cortex. The research team recommends that future investigations adopt standardized, open-source methods for analyzing brain activity.
These demographic differences in nighttime brain waves provide context for the clinical treatment of sleep disorders. Because variations in age and sex relate to exactly how the brain rests, medical interventions might need to be tailored to the individual. Factoring in demographics could help scientists design targeted therapies to support cognitive health as people age.
The study, “Differences in sleep spindles and polysomnography in humans: a meta-analysis on the influence of age, sex, and cognitive ability,” was authored by Diana Campos-Beltrán, Shu Zhang, and Lisa Marshall.