Jean Twenge is lead author of this year's much talked-about study on sleep, smartphones and teens.

The study found that around 40 per cent of adolescents slept less than seven hours a night in 2015 — that's 58 per cent more than in 1991 and 17 per cent more than in 2009. While it's difficult to prove causation, Twenge says there is a correlation between the rise of sleep deprivation in teens and smartphone use.

The findings, published in the current issue of the journal Sleep Medicine, examined data from two surveys of U.S. adolescents conducted over many years. Almost 370,000 participated.

"Some of them said: 'I know I shouldn't have [my phone] with me at night, but I just can't help it.' They talked about it the way addicts talk about a drug," Twenge said.

But the problem goes beyond teens and young adults. "That habit of constantly having the phone in your hand and taking it into the bedroom is very common among adults too," she added.

This week on Checkup, as we talked about technology's effect on our sleep, Twenge shared the details of her research and some tips for parents trying to regulate their child's (or their own) smartphone use.

Duncan McCue: When you started your study on smartphones, teens and sleep, did you find what you were expecting to find?

Jean Twenge: I had been looking at these very large surveys of teens that are done every year in the U.S. for a while. There had already been an increase in the number of teens who were not getting enough sleep and that tapered off for a while – and then came the age of the smartphone, around 2011 to 2012. And sure enough, at the same time, the number of teens not sleeping enough – seven hours of less – started to spike.

DM: Was there anything in the interviews or research you did that surprised you?

JT: I interviewed some of my university students, asking them, "What do you do with your phone when you sleep at night?" And I was surprised that the vast majority said, "I have it right next to me." And I thought, "Why? How can you possibly sleep?" Some said they turned it off, but others said they had it on all night.

DM: Were you surprised that there were that many who bring their phones into the bedroom?

JT: I really wasn't surprised, as many teens and adults are really addicted to this technology – to social media, to texting, to smartphones overall. Most of the young adults I interviewed said that their phone was the last thing that they saw when they went to sleep at night and the first thing they saw when they woke up in the morning. One said it was like a feeling of comfort, she didn't want to have it too far away from her. Some of them said, "I know I shouldn't have it with me at night, but I just can't help it." They talked about it the way addicts talk about a drug or chocolate or whatever they are addicted to.

DM: Do we know how many adults take their phones into the bedroom?

JT: We don't, but I really suspect that this is a trend that goes beyond teens and young adults. That habit of constantly having the phone in your hand and taking into the bedroom is very common among adults too.

DM: As a research psychologist you've been pretty careful to make the distinction between causation and correlation between cell phone use and losing sleep. Can you explain that challenge or dilemma?

JT: We do know that spending more time on the phone is correlated with not getting enough sleep. For example, a third of teens who spend an hour a day on their phone are not getting enough sleep, but among those who spend five or more hours, it's 52 per cent. But you have to ask: is it the phone that's causing less sleep or is it that people who sleep less are using the phone more? It's hard to answer that from the correlational data on individual people, but looking over time, it makes a lot of sense that smartphones became popular at a time when sleep deprivation began to spike. So it makes sense that more people got phones, more people started to spend more time on them and then sleep deprivation happened.

DM: Do you have any suggestions for parents who want to control cell phone use?

JT: The great news here is that you don't get the correlation with sleep deprivation until beyond an hour a day of use. You don't need to take the phone away; it's about moderate amounts of use. In particular, it means putting the phone away at night. A lot of families have a charging station that's away from the bedrooms. Everybody puts their phones there overnight, so no phones in the bedroom – that's a really good rule. Another way (is) to regulate use if your teen goes to bed after you, there are many different apps that parents can put on their teen's phone or their own phone that shuts the phone down during the night.

DM: There are concerns about the rising mental health issues emerging among teens – anxiety or depression – you yourself said that phones might be destroying a generation. What would you say to people who say that that's just analogue nostalgia, that phones are part of life and can be used for good as well?

JT: They absolutely can be used for good, it's a wonderful technology. It's very convenient. It's very helpful, but the key is small to moderate use when we're talking about sleep or mental health. An hour, two hours a day – not a problem, but it's beyond that, it becomes an issue. I'm confused when people say, "Oh, we've always had this worry." That doesn't make it wrong – especially when you're talking about sleep deprivation and mental health. This has nothing to do with what older people think – this is what young people are telling us about how much they are sleeping, how they are feeling. We need to focus on the right now, what teens are saying right now – and they're telling us that they're struggling, and we need to listen to that.