Schmidt, K., Rose, S. A., & Bridger, W. H. (1980). Effect of heartbeat sound on the cardiac and behavioral responsiveness to tactual stimulation in sleeping preterm infants. Developmental Psychology, 16(3), 175-184.
http://dx.doi.org/10.1037/0012-1649.16.3.175
Assessed the effect of heartbeat sound on the heart rate and motor responses evoked by tactual stimulation during sleep. 30 preterm infants, 15 of whom had participated in a long-term intervention program, were tested one day in the presence of sound and another day in the absence of sound. During active sleep, the nonintervened Ss showed a significant cardiac response only in the presence of heartbeat sound. In this sleep state the sound also improved the discernibility of the motor responses for both groups of Ss by reducing spontaneous motor movements. During quiet sleep the sound had no effect on either cardiac or motor responsivity. Additionally, the sound influenced the duration of sleep states for both groups by markedly decreasing the duration of the lengthy 1st active sleep epoch and increasing the duration of the 1st quiet sleep epoch. (34 ref) (PsycINFO Database Record (c) 2016 APA, all rights reserved)