Could sleep be a central regulator for brain disease?

Dr Matei Bolbera, lead author of a recent highly downloaded Journal of Neuroendocrinology article, summarises the paper's key findings. 

Sleep in neurodegenerative diseases: A focus on melatonin, melanin-concentrating hormone and orexin

https://onlinelibrary.wiley.com/doi/full/10.1111/jne.70085

Sleep is one of the most fundamental biological processes in human life, yet for decades it has been treated as little more than collateral damage in neurodegenerative diseases. Patients with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis (ALS), and frontotemporal dementia frequently experience insomnia, fragmented sleep, excessive daytime sleepiness, or circadian disruption. Ironically, these disturbances often affect caregivers and family members even more than the patients themselves. Traditionally, these symptoms were viewed as secondary consequences of neurodegeneration, and ultimately peripheral to the disease itself. Our recent review argues that this perspective is no longer sustainable. Increasing evidence now suggests that sleep and circadian disturbances are not simply symptoms occurring late in disease progression. Instead, they may emerge years, even, before classical neurological symptoms, actively contributing to disease mechanisms rather than passively reflecting them. This shift in perspective has profound implications for how we understand neurodegenerative disorders. At the center of this emerging hypothesis lies: the hypothalamus, a small but critically important brain region regulating sleep, metabolism, hormonal balance, temperature, and circadian rhythms. Within the hypothalamus, three interconnected systems appear especially vulnerable in neurodegeneration: melatonin, orexin (also known as hypocretin), and melanin-concentrating hormone (MCH). These systems orchestrate the balance between wakefulness, restorative sleep, paradoxal sleep (REM: rapid-eye movement), metabolism, and cognitive function. Across multiple neurodegenerative diseases, these neuronal systems progressively deteriorate: In Alzheimer’s disease, disruptions in sleep are tightly associated with amyloid-beta and tau accumulation. Sleep fragmentation, reduced slow-wave sleep, and impaired sleep spindle activity correlate with cognitive decline and disease severity. In Parkinson’s disease, REM sleep behavior disorder can precede motor symptoms by over a decade, suggesting that sleep circuits are affected very early in disease development. And now consider a clinical criteria for severity. Huntington’s disease patients similarly exhibit profound circadian abnormalities and alterations in REM and non-REM sleep. Perhaps one of the most striking examples comes from ALS. Historically considered a purely motor neuron disease, ALS is increasingly recognized as a multisystem disorder involving hypothalamic dysfunction, metabolic disturbances, and profound sleep abnormalities. Recent studies show that patients and even presymptomatic mutation carriers display disrupted sleep architecture long before the onset of paralysis. Experimental mouse models replicate these findings, demonstrating increased wakefulness, fragmented sleep, and circadian impairments across multiple genetic forms of ALS. Why is this important? Because sleep is not merely a period of inactivity. Sleep is an active biological state essential for maintaining neuronal health. During sleep, the brain clears metabolic waste products, consolidates memories, restores synaptic balance, regulates immune responses, and stabilizes metabolic function. When sleep becomes chronically disrupted, these restorative functions begin to fail. Recent evidence strongly supports a bidirectional relationship between sleep and neurodegeneration. Neurodegeneration disrupts sleep-regulating circuits, but impaired sleep may simultaneously accelerate disease progression. Fragmented sleep reduces glymphatic clearance of toxic proteins such as amyloid-beta and tau, promotes inflammation, alters neurotransmitter systems, and impairs synaptic plasticity. In this context, poor sleep is not simply an epiphenomenon, it may actively participate in neuronal vulnerability and degeneration. This emerging hypothesis fundamentally changes the clinical significance of sleep disturbances in neurology and places regular sleep assessment at the forefront of clinical evaluation when these pathologies are investigated. Sleep alterations may become some of the earliest measurable biomarkers of neurodegenerative disease. Changes in sleep microarchitecture, including reduced sleep spindle density, altered slow oscillations, REM instability, or disrupted circadian rhythms, may eventually allow clinicians to detect disease processes long before overt neurological symptoms emerge. This could be especially transformative for disorders like Alzheimer’s disease or ALS, where pathological changes begin many years before diagnosis. Importantly, sleep pathways may also represent therapeutic opportunities. Our review highlights growing evidence supporting interventions targeting melatonin, MCH, and orexin systems. Melatonin supplementation has shown neuroprotective effects in several experimental models, reducing amyloid and tau pathology in Alzheimer’s disease and improving survival in ALS mouse models. MCH-based approaches have demonstrated the ability to restore REM sleep and partially rescue sleep abnormalities in ALS models. Meanwhile, dual orexin receptor antagonists (already clinically used to treat insomnia) have shown promising effects in improving sleep stability and reducing pathological markers in preclinical models of Alzheimer’s disease, Huntington’s disease, and ALS. What comes next for this field is both exciting and challenging? - First, future research must move beyond viewing sleep only through broad clinical symptoms such as insomnia or daytime fatigue. Modern polysomnography (miniature portable) and EEG approaches now allow researchers to study sleep at the level of microarchitecture: sleep spindles, slow oscillations, REM dynamics, and neuronal coupling patterns. These signatures may provide highly sensitive biomarkers of early neurodegeneration. - Second, longitudinal studies are urgently needed. Following individuals over years or decades, particularly genetically at-risk populations, may reveal whether specific sleep alterations predict disease onset, progression speed, or cognitive decline. Finally, chronotherapeutics may emerge as a major frontier in neurology. Combining pharmacological approaches targeting melatonin, orexin, or MCH systems with behavioral interventions such as light therapy, timed feeding, and circadian stabilization could fundamentally reshape how we treat neurodegenerative diseases. The broader message is simple but powerful: sleep may represent one of the earliest and most modifiable biological windows into neurodegeneration. Understanding how sleep, circadian rhythms, metabolism, and hypothalamic circuits interact may ultimately open entirely new avenues for diagnosis, prevention, and therapy. Rather than being a secondary symptom of brain disease, sleep may prove to be one of its central regulators.