Gordon Cairns could tell one of his pupils was feeling tired. In the middle of the day, the boy lay down on the school's baseball court so that he could sleep. It emerged that he was going to bed late and getting up early. His body had no time to repair, recharge and renew itself, and the consequences were felt in every class.
"He just couldn't focus," says Mr Cairns, a part-time teacher in the autistic unit at Govan High School in Glasgow. "He was yawning all the time and lost his sense of where he was. He is definitely happier and livelier now he is getting more sleep."
The turnaround comes courtesy of the charity Sleep Scotland, which is providing "sleep lessons" to four schools in Glasgow, including Govan High's autistic unit. The pilot aims to raise awareness about the pivotal importance of sleep and the sometimes dire consequences of its absence.
Teachers and parents are already vaguely aware that sleep matters, says Jane Ansell, director of Sleep Scotland; they just don't realise how much. But the research is unequivocal.
It shows that a good night's sleep is fundamental to everything from academic ability and behaviour to memory, concentration, sporting prowess, decision-making, creativity and mental health. An Israeli study is particularly telling: losing one hour's sleep a night was enough to reduce a nine to 12-year-old child's cognitive abilities by almost two years the next day.
"It's not like a diet, which may have an impact on your weight within a couple of weeks or months," says Mr Cairns. "Even after one or two good nights' sleep, pupils will feel better for it."
Ms Ansell is in no doubt about the benefits: "Sleep is crucial to repair, growth and the consolidation of knowledge and memory. It's no magic revelation, but we need to ensure the right information about sleep is given to the right people."
At the moment, that message is not getting through. Primary age children need around 10 hours' sleep a night on average, while teenagers need around nine hours. A significant number are way off that figure.
Research for Sleep Scotland suggests that two-thirds of children in Scotland are not getting enough sleep, with some getting as little as five hours a night. "They say they feel sleepy, but they don't make that association with the need to go to bed earlier," says Ms Ansell. "They think that's just the way it is."
Even those going up to their bedrooms at a decent time may stay awake for hours watching television, playing computer games, texting or surfing the internet.
This "media invasion" has a severe social and neurological impact, explains Mandy Gurney, founder of the Millpond Children's Sleep Clinic in London. As well as making them ratty and inattentive, flash images from the computer, TV or mobile phone screens interfere with the body's production of melatonin, a chemical which helps trigger drowsiness.
Over-dependence on technology could mean young people are unable to sleep even when they want to. Turning off their machines at least half an hour before going to bed will help, adds Ms Gurney, as will drinking warm milk or eating yoghurt, which contain tryptophan, an amino acid thought to be a sleep trigger. But some children will still be buzzing when they eventually do go to bed.
"We see a lot of young people who don't know how to switch off," Ms Gurney says. "They finish school at 3.30pm, get shipped around to various activities, do their homework, have a meal and watch an action-packed DVD into the night. There is no transition between their waking and sleeping life."
Fiona Patterson, head of health and well-being at Bellahouston Academy in Glasgow, noticed that her pupils' bed-times were erratic. Some of the school's swimmers were getting up as early as 4am to train.
"A lot of pupils were not getting to sleep until one or two in the morning, and then were expected to get up early in the morning for school," says Ms Patterson.
"Most had no idea that they need nine hours' sleep, and we were ignorant too. I always thought it was eight hours."
Part of Sleep Scotland's remit is to educate teachers, pupils and parents. Just as there has been growing recognition about the importance of healthy eating and exercise in schools, so sleep should be promoted under the umbrella of pupil well-being, it believes.
One of the charity's key messages is that sleep - just like exercise - can act as an anti-depressant. American research shows that teens who get fewer than five hours' sleep a night have a 71 per cent higher risk of suffering depression than those who get eight hours.
But often the benefits of sleep are simply not on young people's - or adults' - radars, Mr Cairns argues. "They don't see it as a problem. Even though there is an epidemic of tiredness in society, people don't view sleep as a priority."
If the effects on performance, memory, growth and physical and mental health do not sway pupils, perhaps the impact on their weight will. A new Canadian study found that inadequate sleep is a risk factor for child obesity, especially among boys.
Another study in New Zealand found that seven-year-olds who have less than nine hours' sleep a night are more than three times as likely to be overweight or obese as those who sleep for longer.
Short sleep duration is associated with reduced levels of leptin, a hormone that regulates appetite, and elevated levels of ghrelin, a hormone that stimulates hunger. The effect on both hormones is closely linked to higher Body Mass Index.
"Sleep has just as big a bearing on obesity as diet and lifestyle," Ms Gurney adds. "When you are tired you think you are hungry. The body craves carbohydrates and sugar, which only offer a short-term fix."
Unlike adults, who often feel lethargic when they are tired, young people are more likely to be hyperactive, fidgety and irritable. It is particularly pronounced among boys from more deprived households, according to a study by the University of Montreal.
Jackie Scott, principal teacher of pastoral care at St Paul's High School in Glasgow, says lack of concentration is a more prominent feature of tiredness among her pupils. "Bedrooms are no longer a place where children can relax and switch off," she says. "Parents think their children are doing their homework or sleeping, when in fact they are playing on their computers."
Every effort to raise attainment and aspiration can be undermined by sleep deprivation, she adds. Boys, "addicted" to their computers, are staying up until 2am or later. They then want to sleep in, miss their breakfast, arrive at school late and find it difficult to pay attention.
She says girls, meanwhile, are getting up earlier than they need to, to do their hair and make-up. Even an 11.30pm bedtime and a 7.30am rise (for an 8.45am school start), falls well short of the recommended nine hours.
Aware of the central role sleep plays in pupil performance, one school has advocated drastic measures. Monkseaton High School, in North Tyneside, has trialled an 11am start to try to avoid "teenage zombies" in the classroom.
The two-hour lie-in had a "profound impact" on learning, according to Dr Paul Kelley, headteacher. Research into the experiment, conducted by Oxford University, showed that pupils' brains work better in the afternoon. It also suggested that teenage brains are wired differently from those of adults and work two hours behind adult time.
With such a packed curriculum, it could be a logistical nightmare for other schools to follow suit. It may also be too much to ask busy teachers to shoe-horn information about sleep into already crammed PSHE lessons, admits Ms Gurney.
"It is a lot to expect of teachers to spot the signs of tiredness, but they should see it as an investment. The more they ensure pupils have a good night's sleep, the easier their lives will be in the long run."
The good news is, sleep is a learned behaviour, says Ms Ansell. Just as sweets and fizzy drinks have been replaced with fruit and water in schools, so good sleep habits can become the norm.
"With motivation, consistency and positive reinforcements, we can change the culture surrounding sleep," Ms Ansell says.
There is everything to play for. Without this key part of the jigsaw, everything else that schools do could fall flat.
- A teachers' pack on sleep, produced by Sleep Scotland, will be available in the autumn. For more information, visit www.sleepscotland.org
- Aim for at least nine hours' sleep a night.
- Keep to a routine: have a set time for the evening meal, bed and getting up in the morning. All contribute to good sleeping patterns.
- Avoid homework, exercise and computer games in the hour before bed. Instead, relax and bathe.
- Switch off the computer, mobile phone and television before having a bath. Listen to music, the radio or read a book.
- Avoid chocolate, caffeine, additives, alcohol and nicotine before bed- time. Replace with a warm milky drink.
- Ensure the bedroom is quiet and dark. It should be a "media free" zone.
Guide to a good night's sleep