One in every six New Zealand teens spends around six hours online each day.

Monitoring screen-time is a modern parents' nightmare.

Anyone who has tried to get grizzly, overtired kids ready for school, or whose teenager throws back coffees before class knows that some young people don't get enough sleep.

While putting your phone or laptop away earlier might sound like common sense, school kids are staying up late doing homework - or on social media - until the small hours.

Are screens getting in the way of a good nights sleep?

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* Parents struggle to limit screen time - study

* Phones used to put kids to sleep

* Children's sleeplessness may be linked to bedtime use of electronic gadgets

A recent international study found that one in six Kiwi 15-year-olds spends more than six hours a day online.

The Programme of International Student Assessment (PISA) tested 540,000 15-year-olds on their education performance, including 4453 in New Zealand.

PISA's student wellbeing report from 2015, published on Thursday, showed that Kiwi 15-year-olds were classed as 'extreme' internet users.

Kiwi teens reported being online for an average of 163 minutes, outside school hours, each weekday, up from 98 minutes in 2012.

The PISA study stated that extreme internet use - more than six hours per day - had a negative relationship with students' life satisfaction and engagement at school.

A third of the 'extreme' group, 31 per cent, had skipped a day's school in the previous fortnight, compared with 15 per cent of 'moderate' users. (1 to 2 hours a day)

Screens are not just delaying bed times - one in four 'extreme' internet users reported being late for school in the two weeks prior to the survey.

An Auckland mother, who did not want to be named, admitted she struggles with her daughter's screen-time.

Her seven-year-old daughter uses her tablet at night to practice her reading or maths skills for 10 minutes before bed each night.

But sometimes she stays on for longer, or watches Netflix, so mum can take some time for herself, she said.

"I do need that break sometimes, but she has a limit in those times too."

She now has to go in and take the tablet from her daughter's room each night because she'd "be sneaky" and keep using it, making her cranky the next day.

"She would be on it all day if I let her," her mother said. And she's not alone.

A 2016 ASG and Monash University survey of 800 New Zealand parents found that 55 per cent felt their children spent too much time in front of a screen, and 48 per cent struggled to limit their child's use of digital devices.

Published data on sleep in New Zealand is limited but growing, PhD student at Massey University's Sleep/Wake Centre, Dee Muller said.

Sleep studies indicate children with screens in their bedrooms went to bed later, slept less on school nights, and had higher scores of sleep disturbance, she said.

Muller, who is investigating the social determinants of preschool children's sleep, said there is an association between short sleep durations, irregular or late bedtimes, poor quality sleep and an increased likelihood of behavioural difficulties in preschool and school aged children. In teenagers, insufficient sleep can cause poor academic performance, and even behavioural and mood problems, she said.

Muller said the rise of screen-based media and portable device use - particularly in bed - poses a challenge for young people to get sufficient, good quality, consistent sleep.

Her advice to parents? "Consider limiting the accessibility and use of technology particularly at night time."

HOW MUCH SLEEP SHOULD CHILDREN BE GETTING?

The National Sleep Foundation and the Ministry of Health recommend the following sleep durations over a 24-hour period.

Preschoolers: between three and four years - 10 to 13 hours.

School age children: between five and 13 years - nine to 11 hours.

Teenagers: between 14 and 17 years - eight to 10 hours.

Young people: between 18 and 25 years - seven to nine hours.