In an era dominated by digital technologies, the concept of digital twins has evolved beyond its traditional applications in engineering and manufacturing, branching out into diverse domains to transform our understanding and interaction with complex systems. We delve into the realm of Human-in-the-Loop Digital Twins (HiLDT), which encapsulates different concepts such as Human Digital Twin (HDT), Cognitive Digital Twin (CDT) and Social Digital Twin (SDT). We present an overview of different forms of digital twins focusing on humans. Furthermore, we explore the opportunities and complexities inherent in HiLDT and identify potential implementation enablers. Through this exploration, we aim to stimulate further research and dialogue on the transformative potential of HiLDT and its impact on human society in the digital age.