ASK any secondary school teacher and you'll hear the same thing. Kids fall asleep in class regularly. Liz Crummey is a learning resource teacher in a south Dublin secondary school. She sees poor sleeping patterns as part and parcel of the chaotic lives that many teens now lead.
"In the last five or 10 years, there's less structure in their lives. There won't necessarily be a parent there when they get up in the morning, they won't necessarily have breakfast, they won't necessarily have dinner. No one actually does the shopping; you eat on the hoof. Very often, there's no actual 'bedtime'."
The last two years have seen a flood of research on just how badly we need sleep, and how little we're getting. Teenagers in particular seem to be suffering something of an insomnia epidemic.
A poll conducted by the Sleep Council in the UK found that one in three 12- to 16-year-olds get only four to seven hours' sleep each night, instead of the eight to nine hours recommended for their age group. US research data suggests only 15pc are getting as much sleep as they need.
Dr Catherine Crowe is a specialist in sleep disorders at the Mater Private Hospital. She says that sleep is as fundamental as food and water.
"It is important that people sleep well, and that's for physical wellbeing, emotional wellbeing and for intellectual wellbeing. A good sleep allows you to concentrate well and be focused, and it keeps your short-term memory functioning properly."
Experts have identified a clear shift in teenage sleep patterns, she says. For younger children, there's little difference in sleep times between weekdays and weekends, but from adolescence on, children tend to stay up later on the weekend and consequently get out of bed later the next day. This is not news, of course, but there are implications that you wouldn't have thought of...
"Say, for instance, they get up at seven o'clock for school, then at the weekends, they get up at 10. That's not a huge shift, lots of kids will do that, but it's three hours nonetheless.
"The body clock takes its cue from getting up time rather than going to bed time. It takes one day to recover a one-hour shift, so if it's a three-hour shift over the weekend, then it takes three days to get back on track."
Parents, she points out, often indulge weekend sleep-ins on the basis that their teen is tired, but by keeping the body clock out of whack until at least Wednesday, this kind of 'shift' will create more problems than it solves.
In addition to the particular risks to teenage sleep, they are, of course, prone to having their sleep disturbed by all the things the rest of us are prone to: alcohol and drugs, caffeine and anxiety.
"Often, that experience of sleeping poorly will create an anxiety," says Crowe. "'God I won't sleep until 2am ... ' So instead of lying there quietly, they'll be anxious, and trying to get themselves to fall asleep. There will be sleep-related anxiety. And, of course, anxiety in relation to school or family can certainly disturb your sleep."
Falling asleep in class is just one of the negative consequences of sleep deprivation. In their recent book, Nurture Shock, science journalists Po Bronson and Ashley Merryman discuss the fact that lack of sleep is directly related to obesity.
"There's at least seven hormones that we know of that are produced during sleep and a number of them relate to metabolism," says Merryman. "These hormones are responsible for hunger and satiation, and they become deregulated if you don't sleep."
Wonder why you get so hungry if you've been up all night? It's because your brain panics.
"Total sleep deprivation for an adult would be the equivalent metabolically of going on a 900 calorie starvation diet. Your brain thinks it's starving to death even though you're actually eating normally."
Merryman is keen to point out that this is not an American phenomenon. Research from New Zealand, Australia, Japan, Tunisia, Hong Kong and Singapore all bear out the relationship between eating and sleep. Adolescents, she says, need nine and a half hours worth of sleep a night.
"For every hour of sleep that they need but they're not getting, their likelihood of becoming obese increases 80pc per hour. One of the researchers we spoke to found that when secondary students fall below eight hours sleep, they double the likelihood of clinical level depression. I'm not talking about a little depression, I'm talking clinical-level depression."
Given this finding, Merryman suggests that all that's wrong with the stereotypical sullen teenager is a late bedtime.
"All of the symptoms of teen moodiness -- short tempers, erratic behaviour, short-term memory loss, impulse control problems, can all be related to sleep deprivation."
Nor does it stop there. Research by the National Sleep Foundation in the US found that students obtaining lower grades went to bed later and had less sleep than their higher-achieving classmates.
Teenage lifestyles sit at the core of the issue. As academic pressure builds and social lives blossom, making time for nine and a half hours sleep becomes an impossible dream.
Tricia O'Byrne is a guidance counsellor at a Dublin school and the mother of a teenage son. She says that the 'always-on' nature of teenage social lives makes it very difficult to fit in something as boring as sleep.
"Teenagers are bringing their phones and laptops up to bed with them. I hear this from a counselling perspective all the time. They're making and receiving texts at all hours and even if they don't receive a text, they're often anticipating one and they're not sleeping."
This results, she says, not alone in poor concentration, but in heightened anxiety the following day.
"My eldest lad is 14 and I know sometimes he'd fall asleep with his phone in his hand. I started to encourage him and the other students I deal with, to leave the phone downstairs."
A Belgian survey of 2,500 teenagers, published in the Journal of Sleep Research found that almost 10pc of 16-year-olds are roused from their sleep several times a week by the bleeping of an unwanted text. A further 20pc are being woken by them up to three times a month. Junk sleep, it appears, is a hard habit to break.
The solution, says Dr Crowe at the Mater, lies in trying to live a more ordered life. Avoid drink and drugs, avoid lengthy sleep-ins at the weekend and keep in good shape.
"People who are fitter sleep better and more deeply," she says.
"Certainly no TV in the room; that's advice for all age groups. Turn off the phone, leave the laptop downstairs. Your room is meant to be a place of security. Keep it like that."