People can spend the same eight hours sleeping and wake up with opposite sensations: some with a rested mind, others still tired. Part of this explanation may lie in a group of cells in the brain stem that are no larger than the tip of a pencil. Scientists revealed that the activity of this structure while awake has an intimate connection with the intensity and level of REM sleep, the nocturnal phase associated with dreams, on August 12, 2026.
The research, led by scientists at the University of Liège, in Belgium, used a 7 Tesla magnetic resonance imaging device to investigate a specific area called the locus coeruleus, located deep in the brain stem. Although previous animal studies have already indicated the crucial role of this structure in regulating sleep and wakefulness, its function in humans remained uncertain. A paper published in the Journal of Biomedical Science now presents evidence of this connection in humans, and the findings could be significant for individuals with insomnia or at risk of developing conditions such as Alzheimer’s and Parkinson’s.
REM sleep, a vital stage long associated with the memory consolidation process, appears to be linked to the balance of locus coeruleus activity during the period in which the person is awake. The way in which this balance manifests itself seems to be directly related to the characteristics of REM sleep, indicating that the dynamics are more complex than simple activation or deactivation.
How the study investigated the relationship between the brain and sleep
Fifty-two adults in good health participated in the study, distributed into two groups by age group: 34 young people, with an average age of approximately 22 years, and 18 older adults, aged approximately 61 years. All volunteers underwent assessments to verify the absence of sleep disorders, psychiatric conditions or other health problems that could influence the research results.
Participants had their sleep monitored in the laboratory throughout the night, using sensors positioned on the scalp that recorded brain electrical activity. In addition, each underwent brain imaging while awake using a 7-Tesla MRI machine, an ultra-high-field technology capable of capturing activity in brain regions as tiny as the locus coeruleus, which measures approximately 15 millimeters long and about 2.5 millimeters wide.
Inside the scanner, participants performed two different mental tasks. One of them involved closely observing a classic optical illusion – an image of a cube that appeared to alternate between two orientations – and pressing a button each time they noticed the visual change. This task was designed to stimulate a constant and continuous mode of activity of the locus coeruleus, known as tonic activity. The second task required participants to listen to a sequence of sounds and press a button when identifying an occasional different tone among the regular sounds. This task was expected to elicit the opposite pattern: sharp, short-lived bursts called phasic activity. Pupil measurements during activities confirmed this configuration.
Based on nighttime sleep recordings, the researchers measured two indicators: the intensity of theta brain wave activity during REM sleep, which is used as a marker of the depth of REM sleep and which previous studies have linked to memory consolidation, and the level of a distinct brain wave pattern that emerges for a brief period just before the body enters REM sleep.
What was observed in brain activity during REM sleep
The results demonstrated a clear connection between locus coeruleus activity during wakefulness and REM sleep quality on the same night. Participants who exhibited stronger locus coeruleus responses in the optical illusion task also showed greater theta activity during REM sleep, indicative of deeper REM sleep. Preparatory brain wave activity before each REM sleep phase was also higher in these same individuals.
A distinct pattern emerged in the sound detection task, especially notable in older participants. In older adults, a more pronounced locus coeruleus response during this activity was associated with a decrease in theta activity in REM sleep. In contrast, this link was not observed in younger adults. It is important to note that the responses of the locus coeruleus in the two tasks were not correlated, which strengthens the researchers’ hypothesis that the activities accessed genuinely different operational modes of the same brain region.
These effects were notably specific to REM sleep. No relevant association was found between locus coeruleus activity and the intensity of slow-wave sleep, which is the measure linked to non-REM sleep and is physically restorative. This points to a particularly targeted relationship between the locus coeruleus and the REM phase.
Why REM sleep quality may affect older people more
The aging process adds complexity to the picture. Sleep quality generally decreases with age, and findings from this study suggest the locus coeruleus as a contributing factor to this reduction. In older participants, a higher peak response during the sound detection task was associated with lower quality REM sleep. The researchers interpreted this as a sign that a locus coeruleus that is more tuned to rapid, reactive daytime responses, rather than a steady, balanced activity pattern, may be related to lower-quality REM sleep.
During normal sleep, the locus coeruleus needs to remain almost inactive for the REM phase to occur. The researchers raised the possibility that the connection between daytime activity patterns and REM sleep quality could weaken with advancing age, although the study did not directly observe the locus coeruleus while sleep occurred.
Beyond the normal aging process, the research team highlighted an even broader clinical relevance. Insomnia is already considered the second most prevalent psychiatric disorder and, although up to a third of the general population reports unsatisfactory sleep, one in every two adults over the age of 50 complains of sleep problems or drowsiness during the day. Additionally, the locus coeruleus is among the first brain regions to show abnormal protein accumulation, which is seen as an early marker for both Alzheimer’s disease and Parkinson’s disease and can appear decades before the appearance of any visible symptoms.
According to the scientific article, the findings “may have implications for the high prevalence of sleep complaints reported in aging and for disorders such as insomnia, Alzheimer’s and Parkinson’s, in which the locus coeruleus may play key roles through sleep.” If the dysregulated activity of the locus coeruleus compromises the quality of REM sleep long before a formal diagnosis, scientists propose that restoring the balance of this region could, in the future, become a target for prevention and treatment. However, this study only focused on observing associations and did not test any treatments.
A notable aspect of the study is that locus coeruleus function was measured only during wakefulness, not during sleep, but even so, its activity aligned with the participants’ sleep quality. The researchers hypothesize that the locus coeruleus behaves in a consistent manner, like a personality trait, suggesting that its pattern of activity during the day may faithfully reflect its performance at night. If future research confirms this theory, scientists could one day identify individuals vulnerable to chronic poor sleep quality without the need to perform overnight brain scans, offering a less invasive diagnostic tool.
For the millions of adults who notice something is wrong with their sleep but can’t pinpoint the cause, part of the answer may lie in a pencil-tip-sized structure located deep inside the brain stem.
