Parents often ask us whether or not to limit their child's use of screens. This used to be about TVs and computers, but now these questions are increasingly asked about the use of tablets and smartphones.

Computers and smartphones can be fun and are a major way that we access information and entertainment. The question is, can they be overused and how much use is too much? Parents can be particularly worried about long term harmful effects on very young children.

We think that the main problem with too much time on screens is that children can miss out on other very important activities, for example, learning language skills, learning how to relate to other people, and having enough physical exercise.

There is plenty of evidence linking various problems with too much TV use, especially in the very young. Studies have shown problems with delayed language development1, sleep problems2 and problems with behaviour3. While many of these studies were carried out before the boom in handheld devices, we believe that, if anything, these later devices are probably more of a problem, as they are so easy to take anywhere and to use anytime.

It's not just a child's emotional and psychological development that might be impaired by too much screen time, their physical health may suffer too. A recent study in the UK found a link between three hours or more of screen time for children and higher body fat, a major risk factor for type 2 diabetes.4 These were children who were not getting enough exercise.

So, how much screen time is too much?

There are no UK guidelines for screen time for a child, but guidelines in the US are for no screen time at all for infants under 18 months and no more than one hour a day for children aged between 2 and 5.5

As well as counting hours, though, we believe that it is important to know your own child and its needs. Start by asking yourself whether or not your child is connecting with other people, in the family and outside; are they getting enough exercise and sleep; have their interests narrowed down just to screen time? If you are concerned about their sociability, their health or having too narrow a range of interests, then it may be time to think about the impact that screen time might be having on them.