Heart rate variability during sleep in Down's syndrome.

We observed heart rate (HR) variability in the sleep-wake cycle of young adults with Down's syndrome (DS) and their age-matched normal controls. Spontaneous HR fluctuations for 10 min during each sleep stage were analyzed by spectral analysis. Only during rapid eye movement (REM) sleep, a middle-frequency (0.05-0.15 cycles/beat) component of the HR fluctuations jointly mediated by the sympathetic and parasympathetic nerves, was significantly lower in the DS than in the control subjects. As the high-frequency (0.15-0.40 cycles/beat) component mediated by the parasympathetic nerve was not different in the two groups, it is suggested that the sympathetic activity during REM sleep is lower in DS than in controls.