Why teenage boys really must stay in bed.

THIS is a message to mothers of teenage boys everywhere who've ever experienced difficulty getting their sons out of bed in the morning - that moment when a maternal plea to "get up now" is met with a muffled "go away, I'm sleeping" from deep beneath the duvet.

The reluctance to emerge isn't laziness. He really is tired. The fact that 90 per cent of teenagers find it hard to get out of bed can be blamed on adolescence. All those hormones and that testosterone surging around the growing body disturbs sleeping patterns.

Thanks to "an authority on teenage sleep", I now know that a hormone tells the body when it's night-time. During puberty, a teenage boy isn't ready for his needed 10 or so hours of kip until 11pm. So when the sun comes up and the world says "good morning", his body thinks it's still the middle of the night and doesn't want to stir.

This is particularly difficult in America where Jesse has to rise at seven in the morning to go to high school. It means that he, like teenagers all over the world, gets several hours less sleep than needed most days.

The second in the Teen Species series had the cameras following half-a-dozen teenage boys for two years. Many fascinating facts came to light.

Did you know that boys grow more in the summer than any time of year thanks to all the eating, sleeping and playing? That son of yours isn't really clumsy - lack of co-ordination results from his body being unable to keep pace with the adolescent changes.

Dominic was a 12-year-old choirboy when we first met him. He'd spent 20 hours a week singing for the past four years. Then his voice broke and he faced life without singing. By the end, he was 14 and couldn't care less about singing as he was playing rugby six days a week.

Some turn to crime. One fifth of all boys in the developed world commit a crime before they're 18. They don't feel they belong, increased testosterone makes them very competitive, and crime gives them a thrill.

Most turn to sex. In the UK, a quarter have sex before they're 16 and in the US, half of them do. Next time your son refuses to leave his bed in the morning, just check he's the only one under the covers.