As the first isochronous substitution study of activity behavior and sleep patterns in young children in China, and also the first isochronous substitution study in the world known to have used a portable multichannel sleep recorder to measure sleep patterns, the present study aims to explore the combined effects of the mutual substitution of different intensities of PA and SB in the same time-space on young children's sleep patterns, and to provide a means of further expanding the depth of the academic value and the breadth of the application value.
In the present study, LPA was found to have a significant positive effect on young children's SE, whereas all of them had a significant negative effect on young children's SFI, and the rest were not significantly associated. This result suggests that LPA may be an effective modality with health benefits in improving sleep patterns in young children, and its mechanism on sleep patterns may be related to circadian rate, body temperature, heart rate, central nervous system, growth hormone secretion, metabolism, immune system, mood, and body composition changes [25–28]. This result is similar to the findings of Ávila-García et al [13], who found that LPA had a significant positive effect on SE in young children and no significant correlation with SOL.
The results of the isochronous substitution model showed that after adjusting for gender, age, BMI, urban-rural category, monthly parental income, and accelerometer wearing duration, when LPA was substituted for SB and MVPA isochronously in 10 min increments, SE increased significantly, and WASO and SFI decreased significantly. This result suggests that increasing the relative proportion of LPA among them and decreasing the proportion of MVPA or SB accordingly, at the same time as the toddler's activity time, is an effective strategy to improve the toddler's sleep pattern. The largest change was found in the mutual substitution with MVPA, followed by SB; the largest change in WASO and SFI was also found in the mutual substitution between LPA and MVPA. This suggests that LPA is the optimal activity behavior for improving sleep patterns in young children and that decreasing MVPA time with a corresponding increase in LPA time produces the highest positive benefit. The findings of this study are not consistent with Laurent et al.'s study [14], which found a significant increase in toddler SE when MVPA was replaced with SB isochronous time [14]. The reasons for this are that, in terms of the study population, in addition to the large sample size attrition rate (68%), the MVPA of the subject toddlers was as high as 113.1 min, which exceeded the guideline recommendation by as much as 88.5%, and the results may not be broadly applicable to the average healthy toddler, and that the study's objective measure of sleep needed to be combined with a sleep diary (questionnaire) to help differentiate between sleep intervals [14],. Sleep results are subjective and subject to error in accuracy. In contrast, the MVPA level of the children in this study was not high (58.7 min), the sample size loss rate was low (14%), and the portable multichannel sleep recorder objectively measured sleep without the need to divide sleep intervals through a sleep diary, so that it could more accurately measure sleep indexes and had higher measurement accuracy. Future interventions to improve sleep patterns in young children could incorporate increasing the proportion of time spent in LPA and decreasing the proportion of time spent in other activities. There may be indirect or direct effects of increasing the proportion of LPA time that could further improve sleep patterns in young children.
According to the results of the isochronous substitution modeling trends, the isochronous substitution benefits are symmetrical, as the LPA isochronous substitution of SB with MVPA time increases, the toddler WASO, SFI continues to decrease and SE continues to increase. Conversely, toddler WASO, SFI continued to increase and SE continued to decrease. This result indicated that the effect of mutual isochronous substitution of PA and SB on sleep patterns in toddlers increases with increasing time of substitution. This study also found that the effect of LPA isochronous substitution for MVPA was higher than substitution for SB in all variables of sleep patterns.The reason for this may be related to the higher probability of MVPA and LPA time switching to each other [29].Individuals have a limited amount of time to engage in MVPA in their lives, and most of them are more inclined to engage in low-intensity activity behaviors after high-intensity activities, which increases the relative proportion of LPA, thus the effect of LPA substituting for other activity behaviors is higher.
4.1. Limitations
Although this study strictly followed the protocol to measure, statistically analyze and create models as scientifically refined as possible, there are still some limitations. On the one hand, the sample was limited to the same province and could not fully represent the overall level of young children in the whole country. On the other hand, this study is a cross-sectional study, and the results of the study indicate the existence of correlation between variables, which cannot fully explain the causal relationship, and may affect the strength of the argument of the causal relationship between variables.