Suckling in the rat: evidence for patterned behavior during sleep.

Electrophysiological recordings showed that 2-wk-old rats are actually asleep 66% of the time while suckling on the dam. The pups are no more likely to be awake while on the nipple than between nursing bouts. Sucking behavior, recorded electromyographically, occurred during sleep as well as during wakefulness. Pups were found to be asleep just prior to the dam's periodic milk ejections (ME) in 100% of 33 recorded instances and were briefly awakened by the ME, but 60% were asleep again within 30 s. These studies demonstrate the embedding of a vital early behavior pattern in the states of sleep.