The World Health Organization (WHO)has released its first ever guidanceon how much time young children should spendusing electronic devices with screens.On Wednesday, WHO announcedthat children under five years oldshould not spend more than one hour a daywatching such devices.Less than that is better, officials say,and children under age oneshould not get any screen time at all."What we are cautioning on is over-useof those electronic screen times with young children,"WHO expert Fiona Bull told reporters.The new guidelines are somewhat similarto advice from the American Academy of Pediatrics.That group recommends children younger than 18 monthsshould avoid screens other than video conferences over the internet.It says parents of children under age twoshould choose "high-quality programming"with educational value.Boys and girls should be able to watch the programwith a parent and understand what they are seeing.The guidelines say that children under fiveshould also be physically active and get enough sleepto help develop good lifelong behaviors.This will help prevent diseases in later life."In this age group of under-5s, it is currently40 million children around the world (who) are overweight.Of that (figure) 50 percent are in Africaand the southeast Asia region," Bull said.The Reuters news agency says that numberrepresents 5.9 percent of all children worldwide.Early childhood is a period of fast physicaland mental development during which behaviors are formedand ways of doing things can be changed, noted WHO.Its guidelines come from evidence in hundreds of studies,many from Australia, Canada, South Africa and the United States."Sedentary behaviors, whether riding motorized transportrather than walking or cycling, sitting at a desk in school,watching TV or playing inactive screen-based games"are increasingly common, WHO said.It added that such behaviors have been linked to poor health.Some groups said WHO's screen time guidelinesfailed to consider the possible benefits of electronic media.Andrew Przybylski is director of researchat the Oxford Internet Institute at the University of Oxford.He told the Associated Press that WHO's screen time advicedeals too much with the amount of screen time.He said it fails to consider the qualityof the material being watched or how it is being used."Not all screen time is created equal," said Przybylski.Britain's Royal College of Paediatrics and Child Healthsaid the study data available was too weakto permit its experts to set any measurefor the best level of screen time.Max Davie is the college's Officer for Health Improvement.He told the AP the restricted screen time limitssuggested by WHOdo not seem proportionate to the possible harm done.WHO did not go into much detail about the possible harmcaused by too much screen time.But the guidelines did note that lack of sleep in childrenhas been linked with increases in extra fatas measured by body mass index.Shorter periods spent sleeping has been linkedwith more time spent watching televisionand playing computer games, it added.