Heatwaves pose increasing risks to human health and socio-economic systems, yet their spatiotemporal organization and underlying synergistic mechanisms remain insufficiently understood, particularly with respect to daytime and nighttime processes. Using a dual identification framework combining absolute and relative temperature thresholds, this study systematically investigates the spatiotemporal evolution of daytime and nighttime heatwaves across China during 1961–2022. A complex network approach is further introduced to characterize the interannual co-variability and interdecadal structural evolution of heatwave activity from a system-level perspective. Results reveal a pronounced interdecadal transition in the early 1990s, accompanied by a fundamental reorganization of heatwave co-occurrence networks. Heatwave frequency exhibits a clear post-transition desynchronization, characterized by a sharp decline in network connectivity and fragmented local clustering, indicating a shift from large-scale, circulation-dominated coherence toward increasingly localized and heterogeneous heatwave occurrences. In contrast, heatwave duration shows an opposite evolution, with significantly enhanced spatial synchronization after the transition. Degree centrality and clustering coefficients increase markedly, and high-connectivity cores expand from coastal regions into inland areas, including North, Central, and Northwest China. This coexistence of desynchronized heatwave occurrence and strongly synchronized persistence suggests an emerging high-risk regime in which heatwaves occur more randomly but, once initiated, tend to persist coherently across large regions. Furthermore, a dual-layer network analysis reveals previously undocumented cross-temporal coupling between daytime and nighttime heatwaves, with pronounced regional differences. The middle and lower reaches of the Yangtze River are more strongly influenced by local processes, whereas northern China is increasingly governed by large-scale circulation control and enhanced regional clustering after the transition. These findings demonstrate that complex network analysis provides a powerful framework for uncovering hidden structural changes in extreme heat events and offer new insights into the evolving risks of compound and persistent heatwaves under climate change.

Under global warming, the frequency and intensity of extreme heat events have increased markedly, having widespread impacts on terrestrial ecosystems and human society. Based on daily maximum temperature observations across China during 1961–2020 and high-resolution ERA5 reanalysis datasets, this study constructs a heatwave synchronization network by integrating event synchronization and complex network theory. The spatiotemporal variations of heatwave occurrence and synchronization during 1961–2020 are systematically investigated. The results reveal a pronounced increase in heatwave frequency across China, with the number of heatwave events during 1991–2020 nearly doubling compared with that in 1961–1990, and the most prominent increase occurring in South China. Although the large-scale spatial pattern of heatwave synchronization remained relatively stable, node connectivity increased substantially, with persistently high values over Northwest China and an evident strengthening trend across North China and the Huang–Huai region in recent decades. Concurrently, heatwave events during the later period were generally accompanied by drier soil conditions, reduced latent heat flux, and enhanced sensible heat flux, indicating strengthened land–atmosphere coupling during heatwave events. These findings provide new insights into the spatiotemporal evolution of heatwave synchronization and contribute to a better understanding of large-scale heatwave hazards under a warming climate.