Purpose <p>Sleep is a key component of biological rhythms, and autonomic regulation across the 24-hour cycle reflects circadian influences on cardiovascular control. Although reduced heart rate variability (HRV) is associated with adverse cardiovascular outcomes, the relationship between habitual sleep duration and parasympathetic function in patients with hypertension remains unclear. This study examined the association between sleep duration and autonomic activity, focusing on the high-frequency (HF) component of HRV as an indicator of parasympathetic regulation.</p> Methods <p>Seventy-four patients with hypertension were classified into a short sleep (S) group (&lt; 7&#xa0;h, <i>n</i> = 28) and a recommended sleep (R) group (7–9&#xa0;h, <i>n</i> = 46). Twenty-four-hour HRV was assessed using Holter electrocardiography, and the HF component was analyzed as an index of parasympathetic activity. Habitual physical activity was measured for one month using a waist-worn monitor, from which physical activity–related energy expenditure (PAEE), moderate-to-vigorous physical activity, and light physical activity were calculated.</p> Results <p>The 24-hour HF component was significantly greater in the R group than in the S group (<i>p</i> = 0.039), even after adjustment for age, sex, dyslipidemia, β-blocker use, and PAEE. Similar but nonsignificant trends were observed during awake and sleep periods (<i>p</i> = 0.078 and 0.074, respectively).</p> Conclusions <p>Achieving the recommended sleep duration was associated with greater parasympathetic activity, independent of habitual physical activity. These findings suggest that adequate sleep is an important behavioral factor for autonomic regulation and may be a relevant target for lifestyle guidance in patients with hypertension.</p>