Early childhood is a period of rapid physical and cognitive development during which habits are formed and family lifestyle routines are adaptable, it said in the guidelines, drawn from evidence in hundreds of studies, many from Australia, Canada, South Africa and the United States.
"Sedentary behaviours, whether riding motorised transport rather than walking or cycling, sitting at a desk in school, watching TV or playing inactive screen-based games are increasingly prevalent and associated with poor health outcomes," the WHO said.
Chronic insufficient sleep in children has been associated with increased excessive fat accumulation as measured by body mass index (BMI), it said.
Shorter sleep duration has been associated with more TV viewing and time spent playing computer games, it added.
"Improving the physical activity, sedentary and sleep time behaviours of young children will contribute to their physical health, reduce the risk of developing obesity in childhood and the associated non-communicable diseases in later life and improve mental health and well-being," the WHO said.
