And they say their findings could eventually produce a drug which could reset the body clock.

This would improve the lives of shift workers and international travellers whose changing sleep patterns cause excessive fatigue and have been linked to health problems.

And a treatment would help those Alzheimer's disease sufferers whose internal clocks go awry.

But other sleep researchers are still sceptical whether a simple drug can be produced to control sleep.

'Essential gear of the clock'

Dr Steven Reppert of Massachusetts General Hospital said the discovery of the protein, known as a cryptochrome, was a "major scientific advance".

He said: "This is a molecular loop that we think is an essential gear of the biological clock.

"You may be able to reset the clock to various phases and reset it very quickly."

The researchers found that the protein is part of a fiendishly complex biological cycle which makes us sleepy and then wakes us up.

Shift workers would benefit from a sleep-control drug

In the morning, they found, two proteins combine to produce a third chemical which, after about 12 hours, has increased sufficiently in volume to trigger another chemical reaction - which stops production and sends the body to sleep.

When the levels of the third chemical fall away sufficiently, the body wakes up and the process starts all over again.

The cryptochrome, which is light sensitive, is the trigger mechanism.

It is well known that exposure to light eventually resets the body's clock from one sleeping pattern to another.

Benjamin Lewin, the editor of the journal "Cell", in which the research was published, said: "Just as a Rolex keeps time according to its own inner machinery and yet can be reset to a new time zone by a turn of a knob, so too can the intrinsic machinery of the biological clock be reset by light.

"The identification that the resetting knob of one clock makes up part of the inner workings of another is certainly surprising and unprecedented."

Sceptics remain

But Professor Jim Horne, of the Sleep Research Laboratory in Loughborough, said he remained to be convinced that humans could chemically alter their body clocks.

He said: "We have yet to find a substance that has an influence on sleep which does not have an influence on other things, such as feeding behaviour, mood or body temperature regulation.

"Some people would say that we are not far off a pill, but I don't think so."

A drug called Melatonin, which has an effect on sleeping patterns, is available in the US but not the UK, as no strict safety tests have been carried out here.