Children who text and e-mail late at night may become depressed, anxious or suffer learning difficulties.

The US researchers said that unlike watching TV, which is a passive activity, sending e-mails and texts involved more interaction and kept the brain busy at the time when it should be shutting down.

The pilot research team from the JFK Sleep Center in Edison, New Jersey, gave questionnaires to 40 people aged between eight and 22, asking about their sleep habits and use of texts and e-mails, reports the Daily Mail.

Those surveyed sent an average of 33.5 text messages or e-mails a night to an average of 3.7 people, adding up to 3,404 text messages per person per month, according to a JFK Sleep Center statement.

The messages were sent from 10 minutes to four hours after bedtime.

In general, boys preferred to surf the web and play video games while girls were more likely to call somebody or send a text.

Across both sexes, emails and texts woke them up once a night on an average and a worrying 77 percent had persistent trouble getting to sleep.

The results also revealed that using technology after bedtime was associated with "high rates of daytime cognitive or mood problems, including anxiety, depression and learning difficulties".

Late-night texts and e-mails led to excessive movement while sleeping, which caused insomnia and even leg pain because the subjects' bodies ended up in an awkward position.