The results are grouped into two primary sections. Descriptive results for each domain (i.e., infant, parent, sleep-wake organization, and so on) are presented first. Age and gender differences, when found, are reported for each of these domains. The second part of the section contains information on the awakening styles. The relationships between these styles and the variables in each domain are presented.
There were no significant differences among the four age groups in infant birth weight, delivery status, health, or gender. All infants were full-term, and all were in good health after birth and at the time of study. Eighty-four percent of the infants were delivered vaginally with no complications. The mean birth weight for all infants was between 7 and 8 pounds (range, 5 lb 4 oz to 10 lb). The general health of the sample was rated very high. Most 3 month olds (97%), 6 month olds (67%), and 9 month olds (67%) were nursing, with 79.2% of 3 month olds exclusively breast-fed. In the 12-month-old group, in contrast, 12 of 18 (67%) were not nursing at all. The age difference between “some nursing” versus “no nursing” was significant (χ32 = 18.2, p < .01). There were no gender differences in nursing status.
Sixty-seven percent of parents reported a stressful event for their child during the day preceding the night of recording on at least one of the four nights. These reports included a range of mild events from pediatric well-baby checks to having a new babysitter. There was a significant association between the presence of a stressful event and gender, with the parents of male infants noting events more often than the parents of female infants (χ12 = 4.66, p < .05). There were no age-related differences in reporting of stressful events. In addition, the majority of the nights that were videotaped were rated as typical nights of sleep by the parents, suggesting that the videotaped nights were typical for this sample and that the families were not unduly affected by the presence of the equipment.
All Beck Depression Inventory (BDI), Parenting Events Scale (PES), and Symptom Checklist-90-Revised (SCL-90-R) scores were within the normal to mild range, confirming the fact that this was not a sample of families with clinical concerns. The overall average BDI score (collapsing across Time 1 and Time 2) was 5.08 (range, 0 to 16). The average Stress scale score was 36.3 (SD = 6.9), and the average Hassle scale score was 34.4 (SD = 9.1). On the SCL-90-R questionnaire, the average Anxiety score was 45.4 SD = 7.9), and the average Global Symptom Index score was 46.9 (SD = 10.1). There were no significant differences in BDI, PES, or SCL-90-R Anxiety and Global Symptom Index scores by infant gender or age group.
provides means and SDs for each sleep-wake variable by age. Several significant differences in sleep-wake organization were found from 3 to 12 months of life. The proportionate amounts of quiet and active sleep changed. The percentage of time infants spent in quiet sleep (QS%) increased (F3,79 = 14.87, p < .01), whereas that of active sleep (AS%) decreased from 3 to 12 months of age (F3,79 = 14.86, p < .01). The longest period of uninterrupted sleep during the night (LSP) also increased in duration with age(F3,79 = 3.12, p = .05).
Means and Standard Deviations Of Sleep-Wake Variables by Age Group
Although the overall amount of wakefulness (AW%) and the number of awakenings during the night did not differ significantly among age groups, the amount of time the infant was removed from his/her crib (OOC%) significantly decreased (F3,79 = 3.43, p < .05). Hence, infants continued to awaken as much throughout the first year of life but were not removed from their cribs for as long at older ages. As listed in , the actual number of awakenings during each of the four nights at each age varied widely. On only a few nights did some infants truly sleep through the night. In general, the mean number of awakenings per night at 3 months of age was equivalent to the mean number at 12 months.
Mean and Standard Deviation of Nighttime Awakenings Across Four Nights by Age Group
A significant interaction effect was found between gender and age group for QS% (F3,79 = 3.67, p < .05). Females averaged higher levels of quiet sleep than males at all ages except at 6 months. The age group × gender interaction was less robust for AS% (F3,79 = 2.36, p < .08). The OOC% demonstrated a significant gender effect as well (F1,79 = 6.21, p < .02). Male infants had a higher average OOC% (mean = 4.56%) than female infants (mean = 2.23%). Female infants also had longer LSP durations than male infants (F1,79 = 16.71, p < .01).
An awakening was coded as “vocalized” if the infant cried, cooed, or babbled and was audible on the videotape recording. An age group main effect on the percentage of vocalized awakenings was significant (F3,79 = 3.37, p < .05), with the 6 month olds exhibiting the lowest level (mean = 78.1%) and the 12 month olds showing the highest average level (mean = 90.1%) of vocalization. There was no significant difference between male and female infants on this variable.
On each of the videotaped nights, the location of the crib was noted. Infants were either sleeping in their own room or in their parents’ room. Crib location did not change across the four nights. Crib location did not differ significantly by gender (χ12= .10; not significant). However, crib location did change with age (χ32 = 16.26, p < .01). A majority of 3-month-old infants had cribs located in their parents’ room (54.5%), in contrast to 5% to 10% of infants at the older ages.
When the infant was first placed in the crib, his or her behavioral state was coded as awake or asleep. Most infants (63.8% for all age groups) were placed into their cribs awake on some nights and asleep on others. There was a general trend across the age groups for the oldest infants to enter the crib awake and the youngest infants to enter already asleep (χ62 = 9.8, p = .20). Restricting the comparison to the 3-month-old and 12-month-old age groups only, a significant difference was found (χ22 = 7.2, p < .05). That is, 12 month olds were more likely to enter the crib awake than the 3 month olds. There were no gender differences based on whether infants entered the crib awake or asleep at any age.
Each time a parent checked on the sleeping infant, it was coded as a “sleep intervention.” The average number of these interventions differed significantly by the age of the infant, with 3-month-old infants receiving more checks per night than the other age groups (mean = 2.2, SD = 2.7; F3,79 = 3.12, p < .05). There were no gender differences in the number of sleep interventions.
Among those infants who vocalized upon awakening and received a parent intervention, the duration from the vocalization to the onset of the intervention was analyzed. The average duration for the sample as a whole was approximately 4 minutes (mean = 245.55 sec, SD = 244.20, range = 1 to 1076 sec). No significant main effects for, or interaction effects between, age and gender were noted. Typically, it was the mother (59.2%) who interacted with the infant for all awakenings during the four videotaped nights. However, in 38% of the families, both mother and father assisted the infant during the middle of the night. The father exclusively provided assistance for only 2.8% of the families.
Analysis of the percentage of awakenings associated with the use of a sleep aid yielded a significant age group × gender interaction (F3,79 = 2.9l, p = .05). The 3-, 9-, and 12-month-old females (58, 30.6, and 56.3%, respectively) used sleep aids more than the males in those age groups. Gender and age were not significant main effects.
As discussed above, each infant’s modal awakening pattern was calculated by identifying the most common type across all awakenings during the four nights of recorded sleep. lists the percentage of infants in each of the four modal types by age group. The most common awakening pattern across all infants was Type 2, in which the infant vocalized but did not receive a parent intervention (43.8%). The least common pattern was Type 1 (2.5%), in which the infant did not vocalize after an awakening and no one interacted with him or her before he or she returned to sleep. The distribution of types across age groups differed significantly, with more of the older infants categorized as Type 2 and fewer categorized as Type 3 compared with the younger age groups (χ92= 18.5, p = .03).
Percentage of Infants in Each of the Four Awakening Types by Age Group
The infants’ modal awakening types were then collapsed into two primary categories, or soothing styles: self-soothing (SS, Type 1 and Type 2) and non–self-soothing (NSS, Type 3 and Type 4). As shown in , there were 37 infants classified as SS and 43 infants as NSS. To confirm that these styles were truly modal, a mean percentage of SS awakenings was calculated for each infant. The NSS group had a mean of 36% SS responses, whereas the SS group had a mean of 79.3% SS responses across the four nights, confirming that the two groups were clearly different.
Percentage of Infants in the Collapsed Soothing Styles by Age
A significant association was found between soothing style (SS vs NSS) and age group (χ32= 11.5, p < .01). Partitioning the χ2 table revealed that proportionately more 3 month olds exhibited the NSS style, whereas more of the 6-, 9-, and 12-month-old infants exhibited the SS style (). Comparison of soothing style by gender revealed a significant association as well (χ12= 7.34, p < .01). Specifically, more males were categorized into the NSS style (69.2%) compared with females (30.8%), regardless of age.
Factors Affecting Soothing Style
Infants with the two primary soothing styles (NSS and SS) were compared by using independent t tests for differences in our previously defined domains. Several of these comparisons were significant. A discussion of each domain follows.
Infant and Parent Domains.
Infant variables, aside from age and gender, included a general health rating by the parent, nursing status, and parental report of stressful events. Overall, there were no significant differences between the two soothing styles on any of these infant variables, with the exception of gender and age, as described previously. It is interesting to note, however, that although more males experienced stressful events as rated by their mothers and more males were categorized in the NSS group, the report of stressful events was not directly related to soothing style. Moreover, there were no significant associations between any of the parental well-being measures and infants’ soothing styles.
More comparisons were significant between the two soothing styles and the sleep-related domains. For instance, LSP differed significantly between the two soothing styles (t78 = 3.93, p < .01). Infants with the SS style had more than 1 hour more of continuous sustained sleep (mean = 367.51 min) than infants with the NSS style (mean = 292.4 min). The total amount of sleep time differed as well, with the SS group exhibiting a significantly longer average total sleep duration per night (mean = 566.35 min) than the NSS group (mean = 521.73 min) (t78 = 3.3l, p< .01). The OOC% differed significantly between styles (t78= 4.31, p < .01), with the NSS group out of the crib for a greater percentage of time than the SS group (5.3 vs 1.4%, respectively). Further, the percentage of vocalized awakenings differed between the SS and NSS groups, with the NSS group vocalizing more often when awake (mean = 89.1%) than the SS group (mean = 81.1%) (t78 = 2.17, p < .05).
The QS% differed significantly between the two styles (t78 = 2.14, p < .05), with infants in the SS group averaging 46.06% of the night in QS compared with 42.63% of infants in the NSS group. However, the AS% or AW% did not differ significantly between the NSS and SS groups.
Sleep Context Domain.
Comparisons between the primary soothing styles and sleep context variables revealed several significant findings as well. Chi-square was used to compare soothing style with sleep-aid use, crib location, and infants’ behavioral state upon entry into the crib. In terms of sleep aid use, there were seven infants who did not use any type of sleep aid, and five of these (71.4%) were categorized as NSS. However, because a large majority of infants (73 of 80) did use some type of sleep aid during the night, the χ2 statistic was not significant. In terms of crib location, 90.9% of infants with the SS style slept in cribs in their own rooms, compared with 64.1% of infants with the NSS style (χ12= 5.71, p = .02).
Finally, the behavioral state of the infants as they were placed into the crib (asleep all four nights, awake all four nights, or mixed) was compared between the SS and the NSS groups (). The χ2 test approached significance (χ22= 5.62, p = .06) and suggested that relatively more NSS infants entered the crib already asleep or sometimes asleep (mixed) compared with the SS group. When the infants with mixed behavior were omitted and only “all asleep” and “all awake” infants were compared, a significant difference was revealed (χ 12= 4.07, p < .05). NSS style infants were more likely to be placed into their cribs already asleep.
Frequency of Infants’ Arousal State When First Placed in the Crib by Age Group
Relationships Between Domains
Other than the infant variables of gender and age, there were few significant associations across sleep-wake organization, sleep context, or infant and parent domains. A few relatively weak associations between the sleep context, sleep-wake organization, and parent well-being domains were noted. In the sleep context domain, sleep aid use was positively correlated with the proportion of vocalized awakenings (r = .26, p < .05). Also, the number of out-of-crib transitions during the night correlated negatively with LSP (r = −.35, p < .05).
In the sleep context and parent domains, the PES Stress score correlated negatively with the percentage of sleep-aid use (r = −.27, p < .05), whereas the SCL-90-R Anxiety score correlated negatively with the OOC% (r = −.26, p < .05).