AbstractThis study was designed to specifically characterize the autonomic phenotype of cardiorespiratory dysregulation during the nighttime in young girls with MECP2 mutation‐confirmed Rett Syndrome (RS), studied in their home environment. Computerized breath‐to‐breath and beat‐to‐beat characterization of at‐home continuously recorded respiratory inductance plethysmography of chest/abdomen and ECG (VivoMetrics, Inc.) was obtained during overnight recordings in 47 girls with MECP2 mutation‐confirmed RS and 47 age‐, gender‐, and ethnicity‐matched screened controls (ages 2–7 years). We determined that although the breathing and heart rate appear more regular during the night compared to the day, young girls with RS demonstrate apparent nocturnal irregularities. Comparing daytime versus nighttime, breathing was more irregular, with an increased breathing frequency (and irregularity), mean amplitude of respiratory inductance plethysmography sum (AMP)/TI, and heart rate and decreased AMP in girls with RS. Comparing girls with RS versus controls during nighttime recording, breathing was more irregular, with an increased breathing frequency (and irregularity), mean AMP/TI, and heart rate. An increased uncoupling between measures of breathing and heart rate control indicates malregulation in the autonomic nervous system, and is apparent during the day as well as the night. This uncoupling may represent a mechanism that renders the girls with RS more vulnerable to sudden death. Pediatr Pulmonol. 2008; 43:1045–1060. © 2008 Wiley‐Liss, Inc.