Sleep and estrogen levels have an impact on neural regulation and are associated with cardiovascular (CV) events. We investigated the effects of estrogen on heart rate variability (HRV) and circadian cycle in spontaneously hypertensive rats (SHRs). Polysomnographic recording was performed in seven male and seven female SHRs during sleep. The electroencephalogram (EEG) and electromyogram (EMG) were evaluated to define active waking (AW), quiet sleep (QS), and paradoxical sleep (PS) stages. Cardiac activities were measured by RR interval of the electrocardiogram (ECG), mean arterial pressure (MAP), and power spectrum of HRV.In ECG, estrogen prolonged the RR interval in total sleep when compared with that at baseline in male SHRs (203.74 +/- 6.61 versus 181.30 +/- 8.06 ms, P < 0.001) and in female SHRs (169.21 +/- 6.43 versus 160.76 +/- 10.66 ms, P < 0.05). In HRV, the estrogen increased the high frequency (HF) in total sleep when compared with that at baseline in male SHRs (1.03 +/- 0.28 versus 0.60 +/- 0.43 ln (ms(2)), P < 0.001) and in female SHRs (0.71 +/- 0.26 versus 0.42 +/- 0.19 ln (ms(2)), P < 0.05).In male SHRs, estrogen increased the frequency of QS (26.50 +/- 4.85 versus 20.79 +/- 5.07, P < 0.01) and PS (25.64 +/- 5.18 versus 20.14 +/- 4.75, P < 0.05) stages when compared with baseline. In female SHRs, estrogen increased the percentage of delta waves in total sleep (79.87% +/- 3.10% versus 76.71% +/- 2.74%, P < 0.05) when compared with that at baseline.In HRV, estrogen leads to neuromodulation by increased parasympathetic tone in all SHRs, suggesting a lower risk to CV events. In sleep analyses, estrogen in male SHRs caused poor sleep quality. In contrast, estrogen in female SHRs demonstrated improved quality of sleep and decreased risk of hypertension.