Stress is a universal human experience, and its impact on sleep has been a topic of interest for both scientists and those who have struggled to find restful slumber after a particularly stressful day. Now, researchers have made significant strides in understanding the connection between stress and sleep by identifying a group of neurons in the brains of mice responsible for regulating microarousals during sleep. This discovery sheds light on the mechanisms behind stress-induced sleep disturbances and may pave the way for improved treatments for sleep-related disorders caused by stress.

Microarousals, which are brief periods of wakefulness during sleep, are a natural part of the sleep cycle for both mice and humans. Throughout a night’s rest, these microarousals are interspersed with periods of deep sleep known as non-rapid eye movement (non-REM) sleep. However, when microarousals become more frequent than normal, they can lead to fragmented, poor-quality sleep and even give rise to sleep disorders like insomnia.

Dr. Shinjae Chung, a neuroscientist at the University of Pennsylvania in Philadelphia, and her team were particularly interested in understanding which brain circuits regulate microarousals and how they are triggered by acute stress. Acute stress, unlike chronic stress, is characterized by sudden and intense events. In humans, acute stressors might include traumatic incidents like car accidents.

To simulate acute stress in mice, the researchers exposed them to repeated attacks from an aggressive mouse. Subsequently, they isolated the stressed mice, leading to a state known as social defeat stress, which continued to affect the targeted mice as they attempted to fall asleep.

During these experiments, the researchers monitored the brain activity of the stressed mice using electroencephalography and electromyography (EEG and EMG) to distinguish between sleep and wakefulness. Additionally, they utilized a brain imaging technique called fiber photometry to observe the activity of specific groups of neurons during sleep. Focusing on the preoptic area of the hypothalamus, a region of the brain known to play a vital role in sleep regulation, they made a significant discovery.

The team found that a particular subset of neurons in the preoptic area of the hypothalamus, known as glutamatergic neurons, was activated during non-REM sleep microarousals under normal conditions. Furthermore, these neurons exhibited increased activity during sleep after acute stress exposure. When the researchers experimentally inhibited these glutamatergic neurons, the stressed mice actually slept for longer durations between microarousals, suggesting the neurons’ crucial role in regulating sleep stability and continuity.

These findings challenge some previous studies that indicated stress might lead to more sleep in mice. However, variations in methodology and experimental conditions may account for these discrepancies. For instance, previous studies returned mice to their home cages to sleep after stress exposure, while in this study, the mice fell asleep in the same stressful environment. Further research could delve into the individual differences in mice’s resilience to stress to gain a better understanding of these inconsistencies.

Although the immediate translation of these findings into treatments for human sleep disorders remains distant, they provide valuable insights into the intricate relationship between sleep and stress. Understanding the specific neural circuits responsible for regulating sleep quality and continuity can serve as a foundation for future research into the broader effects of sleep and stress on human health. As Dr. Ketema Paul, a neuroscientist at the University of California, Los Angeles, notes, “The identification of one circuit out of several potential circuits makes it a lot easier for us to ask that question. To me, that’s the most exciting part of this paper.”

In summary, the discovery of neurons in the mouse brain responsible for regulating sleep microarousals after acute stress exposure represents a significant step towards unraveling the complex interplay between stress and sleep. While the direct applications in human medicine are yet to be determined, this research opens up new avenues for exploring the intricate connections between sleep quality, stress, and overall well-being.