By Tom Bishop BBC News entertainment reporter

Greer tells reality TV host Davina McCall (left) why she left the show

Greer complained that the Channel 4 show behaved irresponsibly towards contestants, whose cold and dark bedroom resembled a "fascist prison".

It was the latest in an increasing number of early exits from reality TV shows, which continue to attract millions of viewers.

Last November, pop star Natalie Appleton quit ITV's I'm A Celebrity... Get Me Out Of Here after being plunged into glass tanks containing putrid waste and live animals.

Police alert

Fellow contestant and ex-E17 singer Brian Harvey soon followed, but not before he was required to sink his teeth into pies filled with insects.

In 2003, actress Danniella Westbrook left I'm A Celebrity, saying her participation in the show was affecting her recovery from drug addiction.

But celebrities are not the only people to face this new, grimmer reality.

Last June, Emma Greenwood was ejected from the non-celebrity version of Big Brother after being told to spy on fellow contestants from a secret bedsit.

Having overheard unflattering remarks from fellow contestant Victor Ebuwa, a fight broke out between the pair when she re-entered the house and police were called.

Brian Harvey quit ITV's I'm A Celebrity after an insect challenge

It seemed a far cry from 2002, when Sunita Sharma and Sandy Cumming quit the Big Brother house. Ms Sharma said she was unhappy, Mr Cumming complained he was "bored".

"We no longer want to watch people sitting around knitting or making cups of tea," said psychologist Dr Cynthia McVey, senior lecturer at Glasgow Caledonian University.

"We have become desensitised to reality shows, so programme makers must devise new ways to keep our attention.

We want to see contestants tried and tested, and a bit frustrated Psychologist Dr Cynthia McVey

"We want to see contestants tried and tested, and a bit frustrated. People now entering these shows should realise that absolutely anything could happen."

Last year's "evil" Big Brother, deliberately devised to be "a lot more oppressive" for contestants, drew nine million viewers - more than twice the previous year - proving that tension could be a ratings hit.

'Public taste'

While Dr McVey agreed that aspects of the current Celebrity Big Brother were "a bit cruel" , she questioned whether programme makers had behaved irresponsibly, as Greer alleged.

"Programme makers must take public taste into account, otherwise they would screen public hangings," Dr McVey said.

"But whether they should be required to do any more than cater for public taste, however awful that may be, is still open to question."

The best approach is not to take any of it seriously Ex-Big Brother contestant Emma Greenwood

Channel 4 and Big Brother production company Endemol declined to comment.

Former Big Brother housemate Emma Greenwood said she and her fellow contestants were required to undergo a series of psychological tests before they entered the Big Brother house.

"We were also told what to expect and warned about the pressures of being filmed all day," she said.

"Of course there were surprises - such as being put into the bedsit - but I thought that was a really good idea. I could see it made brilliant television."

She felt programme makers had been "a bit mean" in isolating her from fellow housemates for three days, but said her dispute had been with contestant Victor Ebuwa rather than Big Brother.

"The best approach is not to take any of it seriously," Greenwood said. "I loved every single minute of it."

Comic Jack Dee (left) won the original Celebrity Big Brother

The Times columnist Caitlin Moran maintains a similar healthy distance from reality television.

"Reality shows are a chance for failed or failing celebrities to be put through test after test lorded over by the Gods - that is, the viewers who vote for their favourites.

"If they pass with humour and grace they are allowed to present a fitness show on ITV1, which is usually the goal they desire."

She believes celebrity contestants are required to face particularly difficult or cruel challenges because they must "prove their humanity" to the general public.

Reality shows can spark healthy debate about issues of morality, Moran adds, but the morality of the viewers themselves is rarely in doubt.