A study published July 13, 2026 in the journal Parkinsonism and Related Disorders recorded heart rate variability (HRV) overnight in patients already diagnosed with Parkinson's disease and found that weaker nocturnal HRV, a measure of the balance between the sympathetic and parasympathetic nervous systems during sleep, was associated with greater autonomic dysfunction, poorer cognitive function and more severe mood symptoms. The paper, posted on PubMed under PMID 42602230 and available in full through PubMed Central (PMCID PMC13473945), describes HRV as reflecting beat-to-beat fluctuations in heart rhythm that can be measured without invasive procedures. A second, separate cross-sectional study, cataloged on PubMed under PMID 42473484, examined cardiac autonomic dysfunction in relation to the severity of cognitive impairment in a distinct group of Parkinson's patients, reinforcing the pattern from a different angle. Together the two reports mark a shift from earlier work in this section, including the UK Biobank analysis that linked weaker heart rate signals to a person's later risk of a Parkinson's diagnosis. That earlier research asked whether HRV could flag people who would go on to develop the disease. These new studies instead ask a different question entirely: among people who already carry a Parkinson's diagnosis, does the strength of their overnight heart rhythm signal correspond to how their thinking and mood are faring day to day. Researchers describe HRV in this context as a potential window onto the autonomic nervous system, the network that governs involuntary functions such as heart rate, digestion and blood pressure, and one that is known to be disrupted in Parkinson's disease. Both papers are observational, meaning they measured associations that existed in patients at a single point or over one recording night rather than testing any intervention or tracking outcomes forward over years. Neither study establishes that changes in HRV cause changes in cognition or mood, or the reverse; the underlying autonomic dysfunction that Parkinson's disease produces could independently affect both heart rhythm control and brain function without one driving the other. The size and duration of the two studies were not detailed in the search results reviewed for this article, a gap that matters for judging how broadly the findings might generalize.