The five “S’s” and the “SNOO” Smart Sleeper—non-pharmacological interventions (NPI) to promote sleep and reduce crying of infants: a scoping review

Introduction

All infants spend a significant amount of time sleeping (1). As they mature substantial changes occur in their sleep/wake cycles which stabilize by 4 to 6 months (1,2). During the first weeks of life, infants wake every few hours, usually to feed, as they develop their circadian rhythm (1). Parental handling including feeding, putting their baby to bed and their response to night wakings impact on the sleep patterns, timing and frequency of feeds of their infants (3,4).

Normal infant crying and sleep patterns

All infants sleep. All infants cry (5). Over time, their pattern and duration of crying gradually change (6). A baby’s crying tends to increase and peak at 6 to 8 weeks of age followed by significant improvement by 3 or 4 months (5,7). On average by 6 to 8 weeks, babies tend to cry 2 to 3 hours per day but may vary from day to day (5). Crying can occur at any time throughout the day, though it usually increases late afternoon or early evening (5,8). In addition, there is considerable variability across different ethnic groups (5). For example, compared to Australian babies, American babies tend to cry more whereas Zambian and Mexican babies tend to cry less (9). There is a variation in the amount of crying by infants depending on their age, ethnicity, underlying well-being, parental handling and many other factors (9).

Contributing factors to infant crying and disturbed sleep

Infant crying and disturbed sleep may be related to several causes involving the infant, parental and environmental factors (10,11). Infants’ factors include their age, temperament, neurobiological development, and non-pathological issues such as hunger or excessive tiredness (10). Other factors may exacerbate the crying, such as an intercurrent infection, cow’s milk protein allergy or gastro-oesophageal reflux disease (12). Parental factors include postnatal depression, stress and anxiety, and a distorted perception of the severity of the crying, all of which may be increased by a lack of family support especially for the mother (11,12). Environmental factors include excessive stimulation from noise, over-handling, length of light exposure, a smoking environment and how settled the infants are in their surroundings (11-13).

Impact of crying and disturbed sleep on family

Excessive infant crying and night wakings can be very distressing, demanding and exhausting for parents and their families, especially the mothers, and may be associated with an increased risk of maternal depression, family discord, psychosocial stress, sudden infant death syndrome, infant asphyxia, and child abuse (6). It also impacts on the father’s well-being (14). In the light of the above, effective management strategies are crucial for reducing infants’ crying and promoting their sleep hopefully relieving parental distress, improving parental sleep and enhancing the family wellbeing (15). Managing crying of irritable infants depends where possible on the underlying cause (16-18).

Medications used to reduce crying and improve sleep

Multiple medications have and continue to be used to settle babies, each with their own complications and with varying efficacy. Dicyclomine hydrochloride (Merbentyl), Scopolamine or phenobarbitone have traditionally been prescribed but are now frowned upon (18). Probiotics are expensive and only seem to mainly help breast fed babies (19) though an earlier study did not confirm that (20). More recently proton pump inhibitors such as Omeprazole, have been used for so-called “silent reflux” despite a lack of evidence that such an entity exists (21). In addition, it is highly questionable if the medications used are efficacious (22). At the same time, they may be potentially harmful with an increased incidence of gastroenteritis, pneumonia and fractures, B12 deficiencies and other metabolic disorders (23,24). In view of the above non-pharmacological management strategies have traditionally included massage, swaddling, warm baths, reducing stimulation, and enveloping the infants in gentle music once one is assured that they are adequately fed and not unwell (25).

The five “S’s”

Dr. Harvey Karp, an American paediatrician, championed the Happiest Baby Method which involved the five “S’s”—swaddling, side/stomach position on the parent, “shushing” sounds simulating “womb-like” sensations, swinging and sucking (15). The five “S’s” were able to induce a calming response (CR)—defined as a universal innate response that that can boost sleep and sooth fussing associated with a drop in the heart rate and increased heart rate variability, in infants by creating an environment that mimics the sensations experienced in utero (15). The swaddling provided a snug feeling, continuous sounds imitated placental blood flow, gentle swinging simulated rhythmic movements created by the maternal diaphragm, and sucking was akin to the swallowing of amniotic fluid in utero (15). The CR improved sleep and rapidly soothed crying infants, unless the crying was related to hunger/discomfort or an underlying pathological cause (15).

The SNOO Smart Sleeper

Dr. Karp developed a smart crib for soothing infants based on three of the five “S’s” (Figure 1) (15). This smart bassinet, known as the SNOO Smart Sleeper, adopted safe swaddling that tucked the baby in (note: not prone, that is on the stomach—which potentially may increase the risk of sudden infant death syndrome), rocked at different speeds depending on the infant’s state of arousal, and created “shushing” sounds of varying intensities to evoke a CR (15,26). According to Karp, infants were sensitive to these simulated intrauterine sensations in the first 3 months of life corresponding to the time when the CR was strongest (27). The SNOO detects crying within 1 minute and responds by increasing the levels of white noise and rocking motions, which incidentally can also be controlled via an internet application on the parent’s phone (15). Once past 3 months, the CR gradually diminishes as infants became better at self-regulation or self-soothing, i.e., how infants deal with a disruption and regain control of their behaviour, and adapting to the extrauterine environment (5,27,28).

Figure 1 The SNOO Smart Sleeper and the five “S’s”. (A) SNOO Smart Sleeper. The Smart Sleeper also rocks and provides shushing sounds relative to the baby’s fussiness and/or crying. The image was obtained from https://happiestbaby.com.au/cdn/shop/products/snoo-mom-watching-sleeping-baby_grande.jpg?v=1584469105. (B) Pictorial summary of the impact of the five “S’s” and the “SNOO” Smart Sleeper on infants’ crying and sleep. The images included were obtained from a Google image search (the source links are as follows). Top left image: https://static.toiimg.com/thumb/msid-72179853/72179853.jpg; middle left image: https://secure.officite.com/user-uploads/article_manager/115964/287258/heroimage0.021779001620084277.jpg; bottom left image: https://happiestbaby.com.au/cdn/shop/products/New-PDP-Images-Asian-couple_640x.progressive.jpg?v=1569281384; right image: https://happiestbaby.com.au/cdn/shop/products/New-PDP-Images-10_640x.progressive.jpg?v=1569277466. Five “S’s”: swaddling, side/stomach position, sucking, swinging and shushing sounds simulating “womb-like” sensations.

Areas of this study

In the light of the above a study was undertaken to explore the current literature to determine the effectiveness of the five “S’s” and the SNOO as non-pharmacological interventions for reducing crying and improving sleep in infants. Additional studies involving the SNOO as a therapeutic agent were also explored. SNOOs are now available in Melbourne and being used in a current study. We present this article in accordance with the PRISMA-ScR reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-23-42/rc).

Methods

Reports describing the five “S’s” and their outcomes were obtained from various sources including the database provided by Dr. Karp in his handbook (27,28). A scoping review was undertaken to determine the efficacy of the SNOO (29). Medical databases, PubMed, Embase and Web of Science were searched during June to September 2021 and included papers ranging from randomized controlled trials (RCTs) to case reports (see Figure 2). More recent reports of work in progress were reviewed but not included in the scoping review. The latter did not have a set protocol but conformed to the following inclusion and exclusion criteria.

Figure 2 Flowchart demonstrating screening process of papers in scoping review of the SNOO Smart Cot. Inclusion and exclusion criteria described under Methods.

Inclusion criteria

• Written in the English language;
• Literature published since 2000;
• Studies involving infants 2 years and under;
• Studies using the SNOO Smart Sleeper or a smart bassinet as an intervention.

Exclusion criteria

• Written in languages other than English;
• Studies lacking clear outcomes;
• Studies with no clear conclusions;
• Any studies that were incomplete and described children older than 2 years were excluded. Appendix 1 lists the search terms and Figure 2 illustrates the relevant Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow chart;

Additional studies to determine if the SNOO had a therapeutic role were briefly reviewed.

Results

The five “S’s”

Öztürk Dönmez and Temel found that using the five “S’s” in their treatment group of 21 mothers and infants, led to a significantly lower mean frequency of waking at night, the mean frequency of feeding, and the mean daily crying duration in infants which persisted beyond the 6-month study period as compared to matched controls (25). Further studies revealed that in combination with simple dietary advice, the five “S’s” were associated with a significant drop in excess weight at 1 year and which persisted to 3 years (1,30,31). Parent satisfaction was found to be significantly improved among mothers with very fussy babies aged 1 year when the five “S’s” were used to placate their infants (32). The four “S’s” (without sucking), were found to be useful for health professionals to help improve infant self-regulation behaviors in the first 12 weeks of life (17,33). Studies adopting the five “S’s” found decreased pain scores on a validated pain scale and decreased crying time among 2- and 4-month-old infants during routine vaccinations, and an effective reduction in pain and length of crying after diphtheria pertussis and tetanus immunization (34,35). A further paper suggested that exposure to womb-like sounds helped reduce hypoxemia and bradycardic events in preterm infants (36). Table 1 summarises many of the studies reviewed.

SNOO Smart Sleeper

Applying the strict selection criteria yielded only 1 article and 1 abstract related to the SNOO, Table 2 summarises the key findings.

In the first study, 7,157 infants sleeping in the SNOO for at least 6 hours per night were compared with the findings of a reference group which were based on a compilation of 13 peer reviewed papers containing normative sleep data of infants asleep in conventional cots (26). Across the 6-month data collection period, infants in the SNOO averaged 1 less waking per night compared to the reference population, had an increased longest sleep period and total sleep duration as well as showed improvement in all sleep metrics that were statistically significant from birth to 6 months (26) (see Table 2). Of note only a standardized comparison was made using normative data which is not the same as a control group and may overestimate the results.

In the second study, 69 infants placed in the SNOO were compared to parental soothing using swaddling, swinging and sounds such as shushing based on the five “S’s” (15). The study suggested that the CR was stronger with parental soothing particularly in younger infants (15) (see Table 2). Additionally, there was reduced infant fussiness and heart rate in both soothing approaches compared to the baseline supine position. There was a stronger CR using the SNOO, while heart rate variability was higher in the parental approach. A reduction in fussiness was comparable with both approaches (15) (see Table 2).

Studies involving the SNOO Smart Sleeper as a therapeutic agent

Conducted in university and/or hospital settings, some of the studies aimed to assess the efficacy of the SNOO in various clinical situations have been summarised in Table 3. One study at the University of Amsterdam highlighted that parental soothing using the five “S’s” in conjunction with mechanical soothing using the SNOO, was optimal in reducing crying and improving sleep in infants (37). A further study found the smart crib placed at the parents’ homes resulted in both babies and parents sleeping better with reduced infant crying (37). Karp described a study at Bellarmine University and Baptist Health Hospital in Louisville, Kentucky, which is exploring the effects of SNOO on breastfeeding and safe sleep in a hospital environment (28). The Children’s Mercy Hospital in Kansas City, Missouri, found that it was feasible to use the SNOO for selected postoperative cardiac infants with parental and staff involvement. That study has been extended as a clinical trial investigating the effects of the SNOO in improving the rate of healing of infants recovering from cardiac surgery (38,39). According to Karp the University of Colorado is studying sleep deprivation, anxiety, and depression among 93 new mothers with a prior history of depression (40). All mothers were provided with a SNOO and monitored over the first six months of the infant’s life. Compared to the 22% to 41% rates of depression reported among a lower risk sample of new mothers during coronavirus disease (COVID), just 17.7% of high-risk mothers using the SNOO, screened positive for postpartum depression, while 18.2% using the SNOO met the clinical criteria for insomnia, which was lower than the expected incidence of insomnia of approximately 46%. Karp also reported that the average rate of anxiety for high-risk mothers using the SNOO was 31.8% as compared to 42.8% reported in a study of low-risk women during COVID (40). He also drew attention to multiple centres reviewing the benefits or otherwise of using the SNOO for infants withdrawing from opiates (28) based on the observations of Paul et al. who found that non-pharmacological interventions have been found helpful (41).

Discussion

In the first 3 months of an infant’s life, up to 20% of parents remain concerned about their baby’s crying and irritability (6). For most infants, there is no apparent underlying cause for the crying (6). While various medications have been used over the years, each with their own problems and with varying efficacy, non-pharmacological strategies such as the five “S’s” have traditionally demonstrated promising results for reducing crying and improving sleep in infants (16) (Table 1).

Happiest Baby method for calming infants, teaches parents of infants the five “S’s” (40). Such parents have gone on to demonstrate a greater ability to calm their infants. Self-regulation by the infants themselves was also better (42-44). Improved infant sleep coupled with dietary advice to the parents, were associated with reduced infant obesity in the first year of life or even later (30,31). This finding may be in part be due to less frequent night time waking reducing the tendency to feed the infant, while improving the mother’s sleep (30). Soothing techniques appeared to reduce pain and the duration of crying of infants receiving routine immunisations (34,35). Exposing premature babies to womb-like sounds, one of the five “S’s”, reduced intermittent hypoxemia and bradycardia (36). Preterm infants exposed to Kangaroo Mother Care had better physiological parameters and developmental outcomes, further evidence of the efficacy of a non-pharmacological intervention (45,46).

Improved infant sleep may result in enhanced parental sleep, reduced parental sleep deprivation and exhaustion which may reduce postnatal depression and anxiety, family distress and even child abuse that can be potentiated by incessant infant crying (16,47).

Using the criteria in the scoping review only 2 studies appeared to have suggested that the SNOO improves sleep and reduces crying in normal infants. Further study however is required to further document its efficacy. Of interest in the paper by Möller et al., in that a varying response was obtained with respect to the CR using the SNOO versus the parental approach with a reduction in fussiness with both approaches but a higher heart rate variability with parental soothing in addition to a stronger CR in younger infants (15). One criticism of the SNOO is that a smart crib is being used to settle a baby rather than being held in mother’s arms. Despite that, its value may be underestimated in that some mothers may be so distressed by the baby’s crying that their repeated handling may in turn further unsettle their baby (11,18).

Studies that have documented the SNOO as an interventional aid in settling babies in various situations appear to be promising, with studies suggesting that it may improve breastfeeding initiation, helping healing from post-cardiac surgery, reducing postpartum insomnia, depression and anxiety in mothers, and settling babies withdrawing from opiates. Such studies become increasingly important during prolonged periods of isolation as seen in the current COVID pandemic (48,49). Of concern is that placing the baby in a cot reduces the time that baby is held closely in the mother’s arms. That however is generally not an issue if the SNOO is only used to help the baby sleep at set times during the day and especially at night. In addition, many fussing and so-called “colicky” babies may settle better if at times away from an exhausted mother (4,17).

Limitations

While considerable work continues in documenting the traditional five “S’s”, only limited data is available to substantiate the benefits or otherwise of the SNOO. Of the 2 studies that were found, one was only an abstract. One was co-authored by Dr Karp who developed the SNOO and may have led to a conflict of interest (26). In addition, a number of the studies incorporating the five “S’s” were described by Dr. Karp (28). It is important therefore to await the results of further studies as to the efficacy of the SNOO but ones without his involvement. The two studies described while having comparable findings only one had a control group (15) while the other used normative data (26). In addition, their outcomes were determined differently. No comment has been made about the cost of the SNOO whether through an outright purchase or by renting for a set period. Concerns have also been expressed that the SNOO may interfere with maternal-infant bonding. The latter needs further careful study. Other smart bassinets such as 4moms mamaRoo Sleep Bassinet, Cradlewise Smart Crib, TruBliss Evi Smart Bassinet were not reviewed and compared with the SNOO which has since obtained Food and Drug Administration (FDA) approval, as only the SNOO was available for our study in Melbourne.

Conclusions

The five “S’s” as well as the SNOO which incorporates 3 of the 5 “S’s” appear to promote sleep and reduce crying of normal infants. Further studies are required to confirm these findings and to determine if the five “S’s” and the SNOO may be used therapeutically for unsettled or unwell infants.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the PRISMA-ScR reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-23-42/rc

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-23-42/prf

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-23-42/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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