We have described a novel device designed to measure the softness of sleep surfaces. Measuring the softness of a variety of surfaces where infants are often put to sleep showed some surprising results. Some infant mattresses were found to be as soft as some adult mattresses. Some adult mattresses were softer than sofas. The cribette playard was as soft as an adult mattress. Of all the infant and adult sleep surfaces we tested, the softest surface we studied was the pediatric hospital mattress. We demonstrated that adding soft bedding to any sleep surface measurably contributed to sleep surface softness.
Some studies have developed similar initiatives for measuring mattress softness. A German investigation of SIDS death scenes placed a 2kg weight at the same location on the sleep surface where the infant was found, and then measured how much the weight sank into the mattress.[18] An Australian group tried to replicate the German mattress measuring device using household items that weighed a total of 2.3kg. Specifically, they placed a dozen CDs tightly bound with cling wrap on top of a mattress surface, and then placed two unopened one-liter milk cartons on top of the CDs.[19] Another SIDS investigation group in the United States measured mattress softness by calculating the area of contact between an infant mannequin head and the sleep surface.[10]
Because Schlaud et. al’s study also used weight to measure mattress firmness, it allows for an easier comparison with our study. Schlaud et. al found that 27 of 41 (67%) scene investigation mattresses had > 14.5 mm indentation. When adjusting for socioeconomic status, nationality, and matching factors, a surface with an indentation of > 14.5mm was significantly associated with risk of SIDS and had an odds ratio of 4.4 (95% CI 1.1–8.7).[18] Using a lighter weight for measurements (2.5lbs ~ 1.1kg), 13 of 17 (76%) surfaces had an indentation > 14.5mm. All the surfaces that had indentation < 14.5mm were infant mattresses (Infant Foam 2006, 7.4; Infant Spring Small 2018, 9.3; Hospital Infant Bassinet 14.0; Infant Cribette Bassinet 2019, 14.2). Infant mattresses are expected to be firmer than all other sleep surfaces; however, we found two of our infant sleep surfaces (2018 Infant Spring, 21.0; 2019 Cribette Playard, 21.4) were just as soft as some adult mattresses and sofas.
Further, we found that folding an infant’s fleece blanket on top of the sleep surface measurably increased softness. Folding the blanket once added 2.3-6.5mm of softness, while folding the blanket twice doubled the softness (4.8-11.6mm) and folding it three times almost tripled the softness (11-21.8mm). Schlaud et. al. found that 88.5% of death scene cases they studied in Germany were using a duvet.[18] They also found that most cases were using a heavier duvet (> 819.5g), not a lighter blanket (500g) found in our study. The use of blankets during infant sleep, however, is common. In a multistate survey conducted in 2010, it was found that 69.3% of infants slept with a light blanket.[16] Many mothers are aware of the dangers of suffocation and strangulation with soft bedding use, like blankets; however, some mothers felt that if the blanket was light and not near the head or neck, that it would be safer to use.[16]
Little has been previously reported specifically on the softness that pillows add to the infant sleep environment. Our study showed that using a firm pillow added 4.0-20.9mm of softness while using a soft pillow added 24.5-46.4mm. This finding contributes to the concern for pillows in the sleep environment posing an especially high risk for SUID. Schlaud et. al found that 41% of SIDS infants were sleeping with pillows compared to only 18% of controls.[18] When controlling for confounders, pillow use was associated with SUID risk with an OR 4.3 (95%CI 1.6–11.6).[18] Besides comfort, many parents use pillows to prevent their infant from falling, especially if the infant is sleeping in unenclosed areas like adult beds or sofas. Parents may think they are making the sleep environment safer, but using pillows creates a particular hazard. [4, 6–8, 13–16, 18] Providing quantitative measures may help parents further understand the risk of adding soft bedding to other sleep surfaces.
We found that sofas and adults mattresses had similar levels of softness, with sofas being firmer than some mattresses. Specifically, we saw that the firmness for adult mattresses ranged from 20.5-34.7mm while the firmness of sofas ranged from 20.9-26.9mm. Sleeping on an adult mattress is more common than sleeping on a sofa; however, sofas have been associated with a significantly more increased risk than adult mattresses because of the increased risk of wedging. [4, 13, 14, 17] One study of SIDS infants in England found that 25.5% of the infants were bed-sharing on an adult mattress while only 6.2% were on the sofa. However, the OR for risk of SIDS controlling for confounding variables was 1.35 (95% CI 0.83–2.20) for bed-sharing and 25.86 (95% CI 6.72–99.47) for sofa sharing. [14]
Few studies have looked into firmness of hospital mattresses. Kemp et al. found that the conventional bedding, which consisted of a foam crib mattress, a spring crib mattress, and two hospital bassinet mattresses, was significantly firmer than the bedding found at death scenes.[10] On the other hand, our study showed the adult mattress (21.8mm) and the infant bassinet (14.0mm) were comparably firm to home mattresses. Furthermore, the pediatric hospital mattress was the softest (36.9mm) of all the surfaces we measured. Ensuring that hospital mattresses are firm is important given hospitalized infants, though typically monitored, are likely at greater risk of SUID given intercurrent illness and the likelihood of other underlying conditions.[4] Also, studies have shown that parents tend to mimic safe sleep practices that they observed while in the hospital.[20, 21] If the infant mattress in the hospital is not as firm as other standard infant mattresses, this may give parents the wrong impression of the correct level of firmness an infant mattress should have.