A new study reveals that age, biological sex, and cognitive ability correspond to distinct patterns in the brain waves that ripple through your mind at night. Analyzing data from nearly 2,000 healthy people, researchers found that the architecture of rest changes with age and is subtly different for men and women. The study was published in the journal Frontiers in Sleep.
As a person spends the night, the brain cycles through different stages. Humans alternate between rapid eye movement sleep, in which they usually have vivid dreams, and several stages of non-rapid eye movement sleep. The latter includes light rest and slow deep rest.
During these non-dreaming stages, special bursts of electricity rush through your brain. These bursts are known as sleep spindles. These vibrational waves, lasting between 0.5 and 3 seconds, are generated deep in the brain by a structure called the thalamus and travel to the outer cortex.
Researchers study these electrical events because they are deeply related to memory consolidation and learning. The bursts represent the brain reorganizing itself in response to new information. Diana Campos-Beltran, a researcher at Germany’s University of Lübeck, and her colleagues wanted to understand how these nighttime patterns differ among the general population.
Many scientists are currently researching 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 the lifespan, it remains difficult to optimize medical interventions.
To build this comprehensive picture, the research team conducted a meta-analysis. This statistical method involves pooling the results of many previously published papers to identify trend lines across large populations. This approach helps smooth out quirks in individual datasets and often reveals patterns that may not be statistically significant in small samples.
The researchers combined data from 42 separate studies into a central database. The final dataset included 1,878 healthy subjects. Each included study had to feature complete polysomnography data as well as quantitative measurements of sleep spindles.
Polysomnography comprehensively records the biological changes that occur during sleep. The original researchers collected this data using sensors placed on the scalp to measure the brain’s electrical activity in a process called electroencephalography. It also incorporates measurements of eye movements and muscle tension to accurately segment the night into specific sleep stages.
The passage of time is associated with distinct changes in the way the human brain rests. The researchers noted that older subjects had decreased density of sleep spindles throughout the night. The amplitude, or electrical height, of these brain waves was also reduced in older adults.
As age increased, the duration of individual sleep spindles decreased. Beyond this short-term electrical activity, older adults also showed a decline in overall sleep quality. Older participants spent less time in the deepest, most restorative rest phase.
These people also experienced much shorter periods of rapid eye movement sleep. In addition to losing deep sleep and dreaming sleep, older study participants also spent more time awake after initially falling asleep. Taken together, these indicators show that sleep becomes increasingly fragmented and shallow as humans age.
Researchers note that biological changes in the aging brain may explain the decline in sleep spindle activity. As the brain ages, both the volume of the thalamus and the integrity of its neural connections to the outer cortex decline. Decreased gray and white matter can cause neurons to fire less synchronously, weakening the electrical signals detected by scalp sensors.
Physical changes on the outside of your head can also affect older measurements. The researchers suggest that as the skull thickens and the distance between the shrunken brain and scalp increases, electrical conductivity may decrease. This may also reduce the EEG amplitude seen in data from older populations.
Biological sex also maps to differences in sleep architecture. Across the data collected, women showed higher absolute sleep spindle power than men. This means that the overall strength of this particular brainwave frequency was greater in women, and this effect is primarily driven by the older subjects in the collected data.
In terms of overall rest quality, women generally had better sleep. Men were more likely to wake up during the night. In the collected studies, men’s total sleep time was also shorter than women’s.
Females spent more time throughout the night in deep slow-wave sleep. Overall sleep efficiency, which measures the percentage of time spent sleeping during bedtime, was lower in male subjects. The researchers suspect that human biology may underlie the differences in sleep patterns between men and women.
Female hormones like progesterone increase signaling of certain neurotransmitter receptors that help create sleep spindles and initiate sleep. Hormonal differences between older men and women may contribute to differences in brainwave activity as the two demographic groups grow older.
The research team also investigated the relationship between nighttime brain activity and waking intelligence. They found that cognitive performance is related to sleep spindles in a highly age-dependent manner. The combined data showed that higher overall sleep spindle power was correlated with higher cognitive test scores.
The total number of nocturnal spindles also correlated positively with intelligence measures. This relationship became subtle when researchers distinguished between fast and slow sleep spindles. Depending on the exact frequency, fast and slow spindles are generated in slightly different areas of the brain.
Humans generate both types of waves, and each type may support different elements of cognitive performance. The cognitive tests used in the original studies varied widely. Researchers in 42 studies used common psychological testing tools to measure reasoning ability, processing speed, and general intelligence.
In adults and older adults, higher density of slow sleep spindles was strongly associated with better cognitive test scores. In children, this particular relationship did not emerge. Instead, children showed a different pattern, including a local association between higher intelligence and increased global spindle density.
Individual sleep spindle length did not correlate with cognitive performance in any age group. The researchers suggest that spindle duration may have more to do with the specific tasks learned that day than with general intelligence. Different learning tasks require different levels of memory consolidation during sleep.
Observational studies like this have inherent limitations. The researchers cautioned that different methods in the original studies could lead to variability in the final combined results. EEG measurement and detection is not a globally standardized process, and each laboratory uses slightly different software thresholds to define sleep spindles.
Additionally, many of the studies published to date have not reported statistically non-significant results. The researchers contacted authors directly to obtain unpublished data, but ultimately some older papers were excluded due to a lack of available measurements. The research team also could not fully explain the physical changes in where sleep spindles appear on the scalp.
Previous research has shown that the location of maximum brain wave activity can change with age. Looking only at standard sensor placement can misrepresent the true power of electrical events if activity shifts to slightly different parts of the cortex. The research team recommends that future studies adopt standardized open-source methods for analyzing brain activity.
These demographic differences in nocturnal EEG provide context for clinical treatment of sleep disorders. Age and gender differences have a bearing on exactly how the brain rests, so medical interventions may need to be tailored to the individual. Taking demographics into account could help scientists design targeted therapies that support cognitive health as we age.
The study, “Differences in sleep spindles and polysomnography in humans: A meta-analysis of the influence of age, gender, and cognitive ability,” was authored by Diana Campos-Beltran, Shu Zhang, and Lisa Marshall.
