The National Sleep Foundation in the US has updated its sleep time duration recommendations, based on analysis of an exhaustive range of research articles.

A total of 18 experts analysed the results of 320 research articles for the purpose – this number was whittled down from an original 2,412 on the basis of the strength of the studies.

The health benefits and also the risks associated with sleep were taken into consideration when the specialists made their recommendations. For example, too little, or too much sleep, can leave a person tired, unable to concentrate, depressed, anxious and in the long term there is an increased risk of diabetes, high blood pressure and obesity.

The new guidelines suggest children aged six to nine years old need 9-11 hours a night, but may get by on 7-8. Teenagers need 8-10 hours, but while 7 hours may be fine for some, sleeping more than 11 hours a day may be detrimental to their health.

One of the study’s experts, Dr Lydia DonCarlos from Loyola University, Chicago, said the circadian rhythm of teenagers naturally shifts to make them feel sleepy later at night and to wake up later. She warned that this is a normal phenomenon and nothing to do with being addicted to social media, as has been reported.

Adults aged 18 to 64 need to sleep for 7-9 hours a night, but some cope on 6. For people over the age of 64, the recommended amount is between 7-8 hours, although some survive on 5 hours sleep.

The recommendations are based on a number of varying studies that differ in their methodology – some were undertaken in laboratory conditions and others were not. The experts did not look at quality of sleep, for example, and whether people wake up in the night and don’t get back to sleep. Sleep architecture – how much rapid-eye movement or slow-wave sleep – was also not taken into account. People who experience higher quality sleep may survive on less than the recommended amounts, the experts added.

“We spend one third of our life asleep, but we know so little about it,” said DonCarlos, emphasising that more research on the topic is needed.