St. James-Roberts and Plewis (1996)
reported that fuss and sleep were the most stable individual characteristics of infant behavior in the first 9 months. Our longitudinal study of healthy, middle class infants examined stability from 6 weeks to 24 months in arousal regulatory properties of sleep, wake and fuss behavioral states, and found additional evidence for cross-age individual stability. Our simple correlational analysis found that awake and content and fuss derived from the Baby’s Day Diary were stable from 6 week to 24 month ages, especially when examining full day and daytime blocks at 6 weeks. The antagonistic relationship found between 24 h sleep and awake & content between 6 A.M. to midday suggests that rates of wakefulness in the early daylight period reflect homeostatic sleep drive.
Using the SEM approach, the 6 week latent variable, wakefulness (constructed by parental scores on awake and content for all time blocks except 12 A.M. to 6 A.M), predicted wakefulness at 24 months (constructed from duration measures). Interestingly, nocturnal wakefulness between 12 A.M. and 6 A.M. did not predict individual differences, perhaps, because homeostatic sleep drive is greatest at that time. Alternatively, parents are probably less aware of quiet waking during the midnight to 6 A.M. circadian time when they are likely sleeping. Nonetheless, these findings from a low risk, healthy cohort argue that daytime wakefulness regulatory properties may be emergent in the neonatal period and phenotypic, as has been found for animal models (Dugovic & Turek, 2001
; Toyota, et al., 2001
), and in adults, where individual differences in sleep-wake parameters are stable, and sleep drive and disorders have strong familial association (Gottlieb, O’Connor, & Wilk, 2007
In toddlerhood, sleep regulation has been established in most children although night waking may recur (Scher & Cohen, 2005
), or still be unresolved (Fukumizu, et al., 2005
). Sleep regulation is strongly promoted as a developmental milestone by parents, and is related to parental practices regarding parent-child sleep proximity during infancy (Hayes, Robert, & Stowe, 1996
; Hayes, Fukumizu, Troese, Sallinen, & Gilles, 2007
). In the current study, parental sleep practices and co-sleeping or night-time proximity to the parents’ bed at 16 months were found to be unrelated to infant temperament or sleep problems. Neither the parent-scored state diary nor parent temperament ratings (i.e. ICQ, TSC) showed a simple relationship to the 16 month sleep habits measured by the SHI. One reason may be that parental sleep practices are not as strongly established or consistent in this developmental window from infancy to toddlerhood as they may be during later childhood. Thus, the link previously found between sleep hygiene, sleep problems and temperament during early childhood and among older children may not yet be evident. Another possibility, and potential limitation, may be the homogeneity of the cohort which was selected for optimal health and family resources. Such middle class, married families likely share very similar sleep practices, as evidenced by the exceptionally low variability in the SHI data at 16 months. Hence, these factors may have limited our ability to detect relationships between temperament and sleep disturbance at 16 months. The optimality of the sample included high rates of breastfeeding and exclusion for infant perinatal reproductive risk which is not typical of community samples. Hence, in the present study the independence of temperament and sleep measures is compelling because the findings cannot be easily attributed to individual differences in child environment or health.
We found that continuity in the temperament dimension of fuss from infancy to 2 years of age was reasonably conserved with the measures used, and this finding refutes the view that early temperament is unstable due to neurodevelopmental affective immaturity (Rothbart, Ahadai, & Evans, 2000
). The methodological convergence of the 6 week ICQ and naturalistic diary observations for fuss, as well as the SEM fuss latent variables for 6 week vs. 16 months and 24 months, supports the idea that fuss, but not crying, is stable in early development. As a temperamental trait, fuss may share construct validity with laboratory measures of negative affect (Kagan, Reznick, Clarke, Snidman, & Garcia-Coll, 1984
) and proposed companion psychobiological markers (Davidson, 1992
; Davidson, 2001
). Importantly, in the fuss SEM analysis, 16 month TSC parental report was predicted by 6 week fuss, but the path was not conserved when 16 months was used to predict 24 month fuss in the SEM model. The 16 month fuss latent variable, constructed from TSC factors attention, self regulation and attachment/emotional functioning symptoms, while predicted by 6 week fuss, may be associating with somewhat different temperamental features than that linking 6 week fuss and fuss at 24 months. The infant fuss SEM latent variable is a reliable link to fuss temperament at both 16 months and 24 months and is therefore a potentially strong predictive marker for a temperamental difficultness trajectory within this developmental timeframe.
These results also suggest that sleep drive exhibits trait-like properties early on that are stable up to two years of age. Hence, individual differences in temperamental style and sleep-wake inclination are evident from infancy, but the interaction of these constructs was not found in any of the models that were tested, and, therefore, we cannot confirm the temperament–sleep association in this sample for these ages.
An important limitation of the study is that all of the measures used were based on parental reports which may be influenced by parental perceptions and biases. Several studies have suggested that maternal stress and anxiety during pregnancy are associated with parental reports of child sleep problems and difficult/fussy temperament in early development (DiPietro, Ghera & Costigan, 2008
; O’Connor, et al., 2007
). In the previous report on this cohort, parents concurred on 6-week assessments for fuss and ICQ composite scores although paternal, but not maternal, psychological distress contributed significantly to the ratings. Mothers’ ratings were more influenced by infant behavioral states (Atella, et al., 2003
). In the present study, maternal ICQ ratings were used for the SEM analyses. The Baby’s Day Diary method has been previously validated through the use of videotapes and voice activated recorders (St. James-Roberts & Plewis, 1996
), and infant difficult temperament is only modestly affected by parental stress with this measure (Darnaudéry, Dutirez, Viltart, Morley-Fletcher, Maccari, 2004
Further work on the sleep-temperament association is needed which uses the prospective longitudinal method and child observations that combine home, laboratory and validated parental assessments in both community and high-risk samples.