Contrary to our hypothesis, the present findings indicate that co-sleeping during the first year of life was associated with the sleep pattern quality of young preschoolers. Specifically, co-sleeping resulted in being independently associated with a poor sleep quality pattern. Accordingly, parents who had practiced co-sleeping perceived their children's sleeping patterns as a severe problem. Compared to parents who had not practiced co-sleeping, they reported their infants slept fewer hours overnight, took longer to fall asleep, and were prone to fall asleep more often in their parents’ arms while eating or in their parent’s bed. One explanation for these results could be that sleeping with the baby may have induced an exaggerated perception of worse sleeping, and so of the BISQ-E items in the parents.

From an evolutionary point of view, co-sleeping reflects the importance of providing close and continuous physical contact within the dyad, which contributes to promoting an optimal infant’s development [8]. Moreover, evidence indicates that co-sleeping is associated with promoting successful breastfeeding [24]. Also, a recent paper states that adequate sleep at nighttime appears to be beneficial for children’s cognitive development [25]. Within this scenario, our study's findings apparently contradict, at least partially, the acknowledged beneficial effects associated with co-sleeping. Moreover, at univariate analysis, we also found that breastfeeding was associated with a poor sleep quality pattern.

However, considering that infants’ sleep patterns are dynamic and change over the first years of life [26], it could be speculated that the quality of the sleep pattern of preschoolers whose parents have practiced co-sleeping may improve over time. In line with this hypothesis, Abdul Jafar et al. conducted a cohort study to evaluate the association between breastfeeding and sleep trajectories from 3 to 54 months [27]. The authors found that breastfed infants showed increased night awakenings compared to formula-fed infants, with an attenuation of this finding from the age of 18 months, suggesting that the more frequent nocturnal wakefulness shown in breastfed infants may be transient.

The effect of co-sleeping during the first months of life on the subsequent sleep pattern of children has not been elucidated yet. In a study on 5-year-old children, Iwata et al. found that co-sleeping at that age did not negatively influence the sleep pattern, both for the child and their parents [1]. Mao et al. conducted a comparative study including babies from 3 to 15 months and found that babies who practiced co-sleeping had more nocturnal awakenings than babies who slept alone [11]. Fernandez-Miaja et al., in a study carried out on children under two years of age in a setting very similar to that of our study, reported a mean value of 10.4 h of night sleep and a mean value of 1.9 h of day sleep time, which are consistent with our findings [28]. On the contrary, the number of nocturnal awakenings found in our study was higher (2.0 vs 1.2). Persistent co-sleeping has also been associated with mother reports of marital and co-parenting distress and lower maternal emotional availability with infants at bedtime [29].

However, when addressing co-sleeping, it must be considered that co-sleeping is valued differently according to cultural backgrounds. In Western countries, infant solitary sleeping is a pursuit aiming at the early achievement of separateness and independent functioning [30]. Accordingly, Mindell et al., in a study including almost 30,000 children, reported that in Asian countries, children share a room and bed with their parents much more frequently than in Caucasian countries [31]. Furthermore, in China, most parents believe that co-sleeping means security, happy family intimacy, and a child’s long-term emotional development [32].

According to our results, a higher percentage of preschoolers fell asleep in their parent’s arms, in their parent’s bed, or eating when they had practiced co-sleeping. In contrast, they fell asleep in their cot or mattress more often if they had not practiced it. We can argue that co-sleeping induces physical dependence in babies while going to sleep, but more research is needed on this association.

Few studies have used latent class analysis concerning sleep habits in preschool children. In a Portuguese cohort of 1092 children, Goncalves et al. identified two behavior patterns at 4 years of age. The pattern with the most sleep difficulties is related to the fact that the mother raises the child without a partner and that the child has no siblings or is not attending daycare [33]. Levenson et al. assess the sleep pattern of children aged 2–5 years, using latent class analysis to find an optimal four-class solution. They conclude that those with a better sleep pattern than their parents have fewer psychopathological difficulties [34]. None of these studies assess the effect of co-sleeping or breastfeeding on sleep patterns. None of these studies evaluate the impact of co-sleeping or breastfeeding on sleep patterns.

Almost all parents who responded to our survey reported that their children had no illness that particularly worried them. However, we consider that babies with some special needs could practice co-sleeping, and consequently, their parents may perceive more difficulties sleeping. This would, however, be the subject of another research design. In this study, we have also not considered specific genetic or family variables regarding sleep that can undoubtedly negatively influence it.

Our study has had several limitations. The first limitation of our research is that the response rate could not be calculated. Secondly, our results have been obtained through a survey based on parents’ perceptions. However, BISQ-E is a well-established, validated instrument to assess sleep quality. Third, since the questionnaire was distributed through schools or pediatric clinics, people from different sociodemographic areas of Spain could potentially have not been reached. Fourth, given the cultural differences between countries regarding infant sleep, it may be difficult to extrapolate our results since what is suitable for some parents, such as sharing a bed beyond one year of age, may be understood as inappropriate for other cultures or frustrating. Fifth, it´s a fact that the sleep pattern of the population studied changes over time from one to two and a half years of age; however, the authors consider that the difficulties do not vary excessively, focusing, for example, on the need to feel accompanied, the number of awakenings or the time taken to fall asleep. Sixth, no minimum number of months of co-sleeping practice was set to answer this question positively; however, language-wise, it is understood that this had been common practice for months. Seventh, in the questionnaire, we asked if there were any illnesses that might be of concern to parents. However, we did not specifically ask about specific chronic illnesses. Although the percentage of parents who responded positively to this question was small, it is possible that a percentage of them had certain significant chronic illnesses that may have affected the baby's sleep and, therefore, our results. Thus, these results should be taken with caution. Future research endeavors, particularly those employing a prospective and more rigorous study design, are anticipated to provide a more definitive resolution to our research inquiry.

In summary, our study has identified that co-sleeping during the first year of life is associated with young preschoolers’ sleep patterns. Further, longitudinal studies are needed to gain further insight into the contribution of co-sleeping on the long-term development of sleep pattern quality. With our results, we do not propose limiting the practice of co-sleeping in parents who decide to do so, as it has proven benefits; however, we want to draw attention to the potential subsequent effects on the sleep pattern. If future studies confirm this association, parents should be aware of it and know how to improve their children's future sleep patterns.