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Background:  Alterations of the autonomic nervous system (ANS) in the chronic stage of ischemic stroke (IS) are not well understood. Heart rate variability (HRV) provides a noninvasive approach to assess autonomic function., Objective:  To compare the HRV parameters during the sleep-wake cycle between patients with IS in the chronic stage and healthy subjects., Methods:  We conducted a retrospective transversal study based on clinical records and 24-hour electrocardiogram (EKG) monitoring registries of 179 patients with a confirmed IS diagnosis and 184 age- and sex-matched healthy subjects. Circadian variation was calculated according to the variation of the total autonomic activity (VTAI) and the parasympathetic activity (VPAI) indexes. Comparisons were performed using nonparametric tests. Multivariable analyses were performed with canonical discriminant analysis (CDA) and a three-way analysis of variance (ANOVA). Statistical significance was established with a confidence level of 95%., Results:  During waking hours, the healthy group exhibited higher variability in the time domain and frequency domain parameters: standard deviation of NN intervals (SDNN, p  < 0.001) and of the average NN intervals (SDANN, p  < 0.001), as well as low-frequency (LF) band ( p  < 0.001). During sleep, the difference was higher in the high-frequency (HF) band ( p  < 0.001), and lower in the low-/high-frequency ratio (LF/HF, p  < 0.001). Both VPAI and VTAI showed less significant difference in IS patients ( p  < 0.001)., Conclusion:  There was diminished heart vagal activity among IS patients, as measured through HRV. During sleep, this is likely caused by an imbalance in the sympathetic and parasympathetic systems shifting through the sleep phases. These imbalances could persist over time in patients with IS, lasting months after the initial injury., Competing Interests: The authors have no conflict of interest to declare., (The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution 4.0 International License, permitting copying and reproduction so long as the original work is given appropriate credit (https://creativecommons.org/licenses/by/4.0/).)