Why circadian disruption and disease can feed on each other
The idea of a two-way cycle is not just that sick people sleep poorly. In Alzheimer's disease, circadian rhythm disruption is reported from the earliest stages, and the disease itself damages brain regions that help keep time. At the same time, disrupted circadian rhythms are thought to aggravate the disease process. Reviews of the clinical and translational literature describe this as a vicious cycle between the two conditions, with clock proteins such as PER, CLOCK, and BMAL1 proposed as molecular links [1]. A separate review of the molecular mechanisms likewise concludes that the interplay between circadian and sleep disturbances and Alzheimer's progression is bidirectional [2].
For neurodegenerative diseases more broadly, a 2023 review summarizes evidence from both human and animal studies showing that circadian disruption and sleep disorders aggravate neurodegeneration, while neurodegenerative disease in turn disrupts circadian rhythms and sleep. The same review notes that circadian disruption and various sleep disorders can increase the risk of neurodegenerative disease, which is the forward direction of the cycle [4].
What evidence supports each direction of the cycle?
The forward direction—circadian disruption contributing to disease—draws on several kinds of evidence. In Alzheimer's research, the review evidence is largely observational and mechanistic: circadian disruption is common in modern life (shift work, light pollution, travel) and is also seen early in Alzheimer's, and clock-related proteins are proposed as targets linking the two [1]. For neurodegeneration generally, the evidence includes human and animal studies showing that disrupted rhythms and sleep disorders are associated with increased risk [4]. These are consistent with a causal role but do not by themselves prove it.
The reverse direction—disease disrupting circadian rhythms—is supported by the observation that circadian and sleep problems are common across neurodegenerative diseases and can occur at multiple levels, and that the disease process itself can disrupt rhythms [4]. In Alzheimer's specifically, circadian dysfunction is described as part of the disease, with irregular sleep/wake cycles reflecting the underlying pathology [2].
A different line of evidence comes from a 2026 study using Mendelian randomization, a genetic method that uses inherited variants to probe causality while reducing some biases of observational studies. That study found that genetic predisposition to shift work causally increased circulating TGF-β1, an immune signaling protein, and SMAD4, with no evidence of reverse causation in the genetic analysis [3]. This supports a one-way causal path from circadian disruption (proxied by shift-work propensity) to an immune marker, but it did not test whether disease changes circadian rhythms. The observational part of the same study found higher TGF-β1 in shift workers, but the association weakened after age adjustment, and neither genetic association survived correction for multiple testing [3]. So this is suggestive, not definitive, evidence for one direction of the cycle.
Where the two-way cycle is less certain
The two-way cycle is best supported for neurodegenerative diseases, where multiple reviews describe bidirectional links based on human and animal studies [1][2][4]. For other conditions, the picture is murkier. In seasonal affective disorder, circadian disruption is associated with the illness, and the gut–microbiome–brain axis is proposed to be bidirectional, but the causal relationships among circadian misalignment, inflammation, and tryptophan metabolism remain incompletely understood, with much of the evidence being associative or from preclinical studies rather than confirmed in patients [5]. That review explicitly notes that microbiome changes in affective disorders may be either upstream modulators or downstream consequences of disease-related behavior [5].
Even in Alzheimer's, the reviews synthesize evidence rather than test causality directly. The Mendelian randomization study provides a template for testing one direction with genetic instruments, but it addressed only shift work to immune markers and did not examine disease-to-circadian effects [3]. So the cycle is plausible and supported by converging evidence, but the strength of causal proof differs by direction and by disease.
Sources used in this answer
Circadian Rhythm Disruption and Alzheimers Disease: The Dynamics of a Vicious Cycle
Review describes a vicious cycle between circadian rhythm disruption and Alzheimer's disease, with clock proteins proposed as molecular links and circadian alignment (bright light therapy plus chronobiotics) beneficial for sundowning and cognitive symptoms in advanced Alzheimer's.
Deciphering the Interacting Mechanisms of Circadian Disruption and Alzheimer’s Disease
Review concludes that the interplay of circadian and sleep disturbances with Alzheimer's disease progression is bidirectional, based on molecular mechanisms of the circadian clock and Alzheimer's pathogenesis.
Circadian disruption and circulating TGF-β1 in workers: evidence from Mendelian randomization and observational data
Mendelian randomization found that genetic predisposition to shift work causally increases circulating TGF-β1 and SMAD4, with supportive observational evidence of higher TGF-β1 in shift workers, but associations did not survive multiple-testing correction and reverse causation was not tested.
Circadian disruption and sleep disorders in neurodegeneration
Review of human and animal studies finds a bidirectional relationship between circadian rhythm and sleep disruption and neurodegenerative diseases, where each aggravates the other and circadian disruption can increase disease risk.
Seasonal Affective Disorder and the Microbiota-Gut-Brain Axis: Circadian Disruption, Tryptophan Metabolism, and Psychobiotic Potential of Lacticaseibacillus rhamnosus GG.
Review of seasonal affective disorder finds circadian disruption is associated with the illness and proposes a likely bidirectional microbiota-gut-brain axis, but causal links remain incompletely understood and direct evidence in seasonal affective disorder is lacking.