Young children can swipe a computer screen but do not have the fine motor skills to play with building blocks, teachers warn.
A teachers' conference heard how some pupils arrive in class tired and unable to concentrate after staying up late playing computer games.
Other children become withdrawn and unable to socialise.
The Association of Teachers and Lecturers (ATL) says tablet computer addiction is a growing concern.
The ATL says children must be taught to use devices, such as tablets and smartphones, responsibly.
The concerns were raised at the union's annual conference in Manchester.
Colin Kinney from Northern Ireland said, while many children displayed highly competent IT skills, they often lacked basic skills.
"Teachers talk of the pupils who come into their classrooms after spending most of the previous night playing computer games and whose attention span is so limited they may as well not be there, or the pupils who wander around at break or lunch glued to their tablets or smartphones.
"I have spoken to a number of nursery teachers who have concerns over the increasing numbers of young pupils who can swipe a screen but have little or no manipulative skills to play with building blocks or the like, or the pupils who cannot socialise with other pupils but whose parents talk proudly of their ability to use a tablet or smartphone."
The ATL passed a motion urging the union to draw up guidance for staff on how best to address the issue.
Mr Kinney went on: "We do not want to deprive our pupils of access to computer tablets. We do want to protect them from withdrawal, poor performance and loss of educational opportunities.
"We are not advocating a ban on devices, we are not stating there are medical risks. We are highlighting rising concerns of social isolation and of decreasing attention span for anything which isn't computer-generated imagery."
Mark Montgomery, also from Northern Ireland, told delegates: "We must recognise and accept that the technology is available and the pupils in front of us are constantly using it.
"Therefore it is our job to make sure that the technology is being used wisely and productively and that pupils are not making backward steps and getting obsessed and exhibiting aggressive and anti-social behaviours.
"In the same way, we can use a brick to either break a window or build a house, digital technology can be used for good or bad and teachers can and should help their pupils make positive choices so they have positive experiences."