Cardiac activity during sleep onset

Alterations in a number of measures of cardiac activity were examined durin g sleep onset in 6 participants over 3 experimental nights. Each sleep onse t was divided into four consecutive phaser: wakefulness, mixed alpha and th eta activity, stage 2 NREM sleep with arousals, and stable stage 2 sleep. T he variables measured were heart rate (I-IR), respiratory sinus arrhythmia (RSA), pre-ejection period (PEP) and T-wave amplitude (TWA). Respiration ra te (RR) was also measured. HR and RR were lower in stable Stage 2 sleep com pared with wakefulness, whereas PEP, TWA and RSA did not change significant ly. During the second and third phases of sleep onset, HR decreased at each transition into sleep and increased following each spontaneous arousal. Th is increase resolved rapidly, with a return to sleep levels by 12 beats aft er the arousal. HR changes are discussed with reference to RSA, PEP, TWA an d the concept of a waking reflex.