By Georgina Kenyon

Tracey Sullivan, 30 of Kew, London always feels terrible in the mornings. She has done ever since she was a teenager.

At least she did until she started sleeping every second night.

"I think I've cracked it. All this time I think I have been running on a 48 hour clock," she claimed.

Chronic tiredness is a common problem. But most people believe they need more sleep, not less.

"When I tried to sleep properly each night, about six or seven hours, I could never wake up properly. When the alarm went off at 7am, I could hardly drag myself to work each day and I would not wake up totally until late afternoon," said Tracey.

"For 15 years I could not break this pattern of tiredness - I thought maybe I was ill.

"I was in a kind of daze for most of the day. As a teacher this is not ideal."

New sleep pattern

Tracey now sleeps about eight hours each alternate night and has a short nap of about two hours on the others.

"I feel much more alert and invigorated. I no longer get out of bed feeling like death on legs."

So is it possible that some people operate on a different rhythm to the usual 24-hour cycle?

According to Professor Ian Hindmarch, Head of Human Psychopharmacology at the University of Surrey in Guildford: "Many people like to claim they only need very small amounts of sleep. Four hours a night for instance.

"But we have found that when such people come in for tests, we find in most instances they are sleeping during the day and not admitting it, or are sleeping more than they think at night."

Thatcher

Former prime minister Margaret Thatcher was famous for requiring little sleep.

But Professor Hindmarch said: "I have many pictures of Margaret Thatcher asleep during the day.

"I would need to observe Tracey's sleep patterns to check this was not the case."

But Tracey has no opportunity to nap on the job. Her fellow teachers would quickly see to that.

The school ground has never been known as the most forgiving environment. And her husband can vouch for her two hours' sleep on alternate nights.

However, her GP suggested perhaps it was her state of mind. Being a rather highly-strung person, she may keep her self up at night worrying one night, while the following she falls asleep exhausted.

Tracey, however, dismisses this explanation.

"How come I feel far more awake every day?"

Underestimation

Professor Jim Horne, Director of the Loughborough Sleep Research Unit, Loughborough University, agreed it was true that true that most people underestimate how much they sleep.

"Of course, people can go without sleep for up to three days when under tremendous stress like in military combat.

"Studies have shown however that people who only need very small amounts of sleep often show signs of mania."

Tracey is not overly pleased with such an explanation.

"But it's up to each individual to find out how much sleep they need and when," said Professor Hindmarch.

Many scientific studies abound on what is the ideal sleep time.

Professor Hindmarch estimates that anything between three and 12 hours a night is within the normal range.

Professor Horne added: "Mood swings and exercise will also affect sleeping times. Exercise at night will make it harder to sleep and depression will increase sleep time."

Tracey believes this could be an explanation for her new sleep routine.

"I have always felt that I am very happy every second day and really quite deflated in between days.

"Maybe when I up upbeat in the day, I am up at night and active."

High achievers

Tracey's mother suggested her daughter had always been a bright child and as she had read reports in the press that high achievers often do not need much sleep, this could be why.

But Professor Horne is having none of it.

"Parents often like to think that if their baby only sleeps half as much as its peers its because the child is intelligent. I'm afraid to say this is not the case," he said.

"It's an urban myth to make parents feel better about feeling permanently tired."

It is not yet fully understood why humans need to sleep each night.

Some studies suggest that sleep rids the body of toxins, others that it is needed to consolidate memory or is necessary for creative thought.

Some scientists have also suggested it is simply biological. It was safer to be in the cave and sleeping than being out in the dark and possible eaten by predators.