So this is where you come when nothing else has worked. Light, airy rooms; soothing pastel colours. A spectacular view of the river, the Houses of Parliament.

You sleep on a comfortable fold-down couch. Your child sleeps next to you, in a hospital bed, attached to 20 leads and sensors. A microphone records every sound; an infrared video camera observes the scene.

Next door, in the morning, technicians will analyse the data collected through the night: brain activity, eye movement, air flow, carbon dioxide level, leg movement, snoring volume.

This is where people come when they have tried everything and their child is still not sleeping enough, or not well enough: Hotel Sleep at the Evelina children's sleep disorder service, Guy's and St Thomas's hospital in London.

An astonishing number of parents – about one in five – do not think their children are sleeping as they should. We are talking, says the service's director, Professor Paul Gringras, about "a very general dissatisfaction".

But it is a long way from that dissatisfaction to the beds of Hotel Sleep. First, what constitutes enough sleep – and normal sleep behaviour – covers a very wide range.

From the Avon longitudinal study, which has followed the health and development of 14,000 children in the Avon area since they were born in 1991 and 1992, we know, for example, that the average one-year-old sleeps just more than 11 hours each night.

"So some mothers will be tearing their hair out because their child is only sleeping nine hours, others if theirs are sleeping 13 hours," says Gringras. "Both are normal. They fall inside the bell curve, within what is expected. There are short sleepers and long sleepers."

It may be more useful, he says, to just look for changes in the sleep pattern of an individual child: "If that changes – if they are waking more at night, there's a sudden change in how long they sleep, or physical signs such as poor growth – that would be a sign of something to seek advice about. But sleep is one of those areas where the line between normal and abnormal can blur."

It is also the case that a "huge proportion" of sleep problems in young children are essentially behavioural, Gringras says. By which he means: "If a child falls asleep on dad's lap, sucking a bottle, with him stroking their hair," he says, "then when that child wakes up during the night – which all children do, perfectly normally, several times each night – they won't go back to sleep again unless they're on dad's lap, sucking a bottle, with him stroking their hair."

The take-home lesson? "Don't do anything at bedtime that you are not prepared to do five times a night, including at 3am." Which is, of course, easier said than done.

But with determination and a favourable wind, most sleep-related behavioural problems can eventually be cracked, Gringras says. The trouble is, not all parents have the determination and not all winds are favourable. "When you've got two parents and two children in separate bedrooms, it should normally be doable," he says. "Sadly, what's more common might be a single mum, sharing a bedroom with her kid, and the neighbours have threatened to apply for an Asbo because of the crying. That's not so easy."

It is important to resolve, though, because sleep is vitally important – particularly for children. "Not only do we consolidate memory and learning when we sleep," says Gringras, "but poor sleep impacts on areas like growth, immunity and blood pressure. It's no exaggeration to say that, every day, research is linking another area of physical health to sleep."

Some of the strongest effects of sleep – or lack of it – are in learning and behaviour. Gringras cites a fascinating study by an Israeli clinical psychologist, Avi Sadeh, who took a class of 80 10-year-olds and measured how much, on average, each child slept at night.

He then asked the parents of half the children to encourage them to go to bed an hour earlier than usual and the other half an hour later, for the next three days. The children did computer-based learning tasks – designed to reflect classroom-based behaviours, learning and achievement – before and after these small bedtime changes. The results were startling.

"There was a very, very clear difference between the two groups," says Gringras. "The differences in neurobehavioural function were equivalent to those acquired in two years of learning." In other words, those who slept less than usual found themselves two years behind their classmates – after a matter of days.

Even more intriguingly, similar studies have shown that ADHD-type behaviour appears in many of such sleep-deprived groups. Unlike adults, Gringras says, children who are tired will not only never say so, but "will seek stimulation to stay awake. They'll bounce off the walls, get labelled naughty. Some researchers believe a proportion of ADHD-like problems could be down to sleep issues."

Lack of sleep, or poor sleep, can have other consequences on children's development. "If our sleep is fragmented, or we get too little," says Gringras, "we produce more ghrelin, but less leptin – it's thought that these changes will drive our appetite for carbohydrate-rich foods. We are seeing an explosion in numbers of severely obese children.

"They have breathing problems, their sleep is fragmented. Although they may appear to sleep 10 hours a night, in the lab we see they actually wake up 50 times an hour. And so they are hungry. Sleep is part of a vicious cycle that drives obesity."

Gringras also sees a lot of children seemingly welded to their screens. The trouble with screens – TV, computers, consoles, mobile phones, tablets – is that our sleep patterns are governed by light and hormones.

When natural light starts to dim at the end of the day, our pineal gland produces a hormone called melatonin, which triggers the body clock to "open the gate that allows us to sleep," Gringras says.

Unfortunately, exposure to bright light – which includes most screens, however small – will halt the natural production of melatonin and so prevent sleep. Telly just before bedtime, half an hour on Facebook or similar, even a spot of under-the-bedsheets texting: all will prevent sleep.

"I increasingly see in the clinic children who are effectively switching off this sleep hormone by exposing themselves to a bright light source – particularly the blue-green spectrum from computers and TVs," says Gringras. "Even a very brief exposure – two minutes – towards the end of the evening can put sleep off by as much as two hours. A computer screen will do it if you sit close enough. Smartphones and tablets are just as bad: smaller screens, but you hold them very, very close to your face."

Sometimes, parents come to the clinic to ask for melatonin on prescription. "It's not uncommon to discover that the child has a 46-inch TV screen at the foot of the bed. In cases like that, we can only say sorry, no can do," says Gringras.

It is not just the child who suffers as a result of inadequate sleep. A child not sleeping well can have an impact not just on the individual but on the whole family. Generally, families come to Gringras because their child is either not falling asleep, is behaving abnormally – waking up, suffering from night terrors, sleepwalking or talking – during the night or seems abnormally tired during the day.

Noisy night-time behaviour, such as headbanging [a silent, rhythmic bashing of head against pillow], is more common than people think, affecting 6% of young children (but only 1% of 12-year-olds, so it does get better) and actually a sign that a child is sleeping very deeply. But it can disrupt the whole family.

"Get it right, and there are far wider benefits," says Gringras. "The whole family can be transformed. Parents are less tired the next day: dad may be less likely to have a car accident, big sister may get the sleep she needs to do well in her GCSEs. Good sleep can have an impact way beyond the individual."

Paul Gringras's top sleep tips for children (and their parents)

1. Create a good bedtime routine and environment

• No screen time (TV, iPad, smartphone, laptop) for at least an hour before bed.

• Wind down properly (bath, book, time with Mum and Dad).

• Good blackout blinds.

• Good bedtime expectations ("We expect you to stay in your bed, and if you do we may even reward you").

• Soft, continuous-play music often works; it masks traffic and sibling noise, and if they go to sleep to music they will go back to sleep to it too.

• If the child demands a nightlight, it should be dim or red. The brain is not so sensitive to red light.

2. Encourage "self-soothing"

It is perfectly normal for a child to wake up four or five times in the night, but they should be able to "self-soothe" – go back to sleep on their own. "Concentrate on what children need to get to sleep at the beginning of the night, and remove everything that won't be there when they wake up in the middle of the night." So:

• Cuddle by all means, but not until your child falls asleep.

• Sit and talk, but likewise.

• Break any association between milk or food and falling asleep.

3. Behavioural problems

• Nothing beats a sleep diary. Both parents should write down what they do when putting the child to bed each night, and when trying to get them back to sleep. Simply writing it down will allow you to realise the patterns that have developed, and you will often be able to solve the problem.

• Cold turkey, or less dramatic variations on it, is harsh but invariably effective. If you're strong enough, simply leave your child to cry; Gringras describes sleep units in Australian hospitals where trained nurses are on hand specifically to help mothers not to go in to their crying babies.

• If you can't do that, opt for a more gradual approach, spending a little less time with your crying child each night.