Quiet sleep, neonates

Phasic muscle twitches are one of the major REM phasic activities in neonates and also comprise prominent neonatal behavior. A similar feature is found not only in rats but also in other rodents and humans. The period that shows frequent muscle twitches is the first 4 weeks in the rat (4), the first 40 days in the kitten (8), and the first 8 months in the human newborn (13). In humans, this feature is more typical in the premature fetus (14). This activity appears primarily during REM sleep but also in a small portion of NREM sleep, i.e., half-activated sleep (4). The rate of muscle twitches is only 1.5/min and 0.3/min during quiet sleep compared to the rate of 7.5/min and 3/min, respectively, during REM sleep at the same age in PN 10 and PN 20 kittens (8). The number of phasic events dramatically decreases as animals mature (8,9,13,15). It is of interest to note that the dramatic reduction of phasic activities in REM sleep is associated with the increase of wakefulness. 

The decrease of REM sleep without a corresponding increase of wakefulness might be one of the most important features in the short-term effects of neonatal RSD. This fact indicates the immature wake generator or a low waking capacity in the neonatal period. A similar observation was first reported by Mirmiran et al. (61) and described as an increase of NREM (quiet) sleep during neonatal RSD by CLI, without emphasizing its importance. 

Hilakivi (ref. 128) found that in rats prenatal alcohol exposure during the entire period of pregnancy resulted in less active sleep, more wake and a more frequent interruption of the quiet sleep state by waking episodes on neonatal age. Human newborns with FAS may show abnormal EEG profiles and sleep disturbances such as reduced REM sleep (ref. 25). 

In adult humans and animals, chronic intermittent hypoxia has a facilatory effect manifest by enhanced responses to acute hypoxia (52,53), which appears to involve a serotonin-dependent mechanism (53). Both facilatory and inhibitory effects have been observed in neonatal animals (48,50,51). In addition, state of arousal has been shown to be a factor in the response to repetitive hypoxia in the neonate. In newborn lambs, for example, repetitive hypoxia rapidly became ineffective as a stimulus during active sleep but retained its responses during quiet sleep (49). 

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