Tiredness is a common complaint in the modern world. Shift work, long commutes, offices shut away from natural light, hectic social lives: all of these factors can wreak havoc on our energy levels.
But what if the exhaustion becomes so severe that it stops you from looking after your children, going to work -- or even getting out of bed?
For decades, the medical world has wrestled with the idea of defining such extreme fatigue, along with a host of other debilitating symptoms such as muscle pain, digestive problems and inability to concentrate, as a real disorder.
In Europe, we refer to it loosely as ME (Myalgic Encephalomyelitis); in the States they use an umbrella term called CFS (Chronic Fatigue Syndrome).
Yet there has been bitter debate and division between sufferers, the sometimes disbelieving 'healthy' public and doctors on the symptoms and causes of such a syndrome -- or if indeed it really exists as a purely physical condition.
As sufferer Marie Coffey explains, ME feels like one is constantly living with an excruciating hangover and it can be difficult for people to take it seriously as a physical ailment.
There was some excitement then last week when Science journal published a study that seemed to establish a link between ME/CFS sufferers and a recently discovered virus called XMRV.
"Since the original Science paper was submitted, we have continued to refine our test for XMRV and have surprisingly found that 95pc ME/CFS samples tested positive for XMRV antibodies in the plasma," said Dr Judy Mikovits, who led the research at the Whittemore Peterson Institute (WPI) in the University of Nevada, Reno.
"This finding clearly points to the retrovirus as a significant contributing factor in this illness," she added.
Tentative though the findings are, they seem a vindication of what sufferers have been saying all along: it's not all in our minds.
Because of the difficulty of diagnosing someone with ME, and the reluctance to see it as a definite syndrome anyway, there has been a tendency to dismiss sufferers as hypochondriacs.
In fact, when Dr Daniel Peterson of the WPI started a practice in Nevada in the 1980s and began diagnosing a large number of patients coming down with a new type of chronic fatigue illness, the media derogatively tagged it 'yuppie flu'.
The implication was that it was a type of psychological burnout particular to the social circumstances of Dr Peterson's affluent patients in the Lake Tahoe area.
This was incorrect, as the illness was being documented across lots of other areas, but it stuck for several reasons, some of which were to do with the health insurance industry in the US who didn't like the idea of categorising a new illness that they would have to pay out for.
Declan Carroll of the Irish ME Trust explains that the symptoms being diagnosed by Dr Peterson were not actually new to Europeans who had been loosely coining the term ME since a study in a London hospital in the 1950s.
A population study taken by a Prof Leonard Jason in the US of figures for CFS worldwide concluded that it affects a mean of 0.3pc of the population. In Ireland, that figure translates to around 12,000 people.
Questionnaires submitted to Irish GPs by the Irish ME Trust came up with a similar number, although the Trust admits that this is not conclusive.
"Basically what the uncovering of this new virus has done is to further demand a proper diagnostic approach," says Declan Carroll.
The best way forward for ME sufferers here -- at least to diagnose them -- is for doctors to apply Canadian guidelines which set out a number of fixed criteria for diagnosing the illness.
"What happens fairly regularly," says Declan Carroll, "is doctors are busy, they can't see anything untoward in regular blood tests, and they pass the patient onto a consultant that deals with the prevalent symptom, say a person with muscle pain gets sent to a rheumatologist.
"Otherwise they take the view: you have muscle pain, you're tired all the time, it may be burnout, stress, so go exercise and I'll give you an antidepressant. And that is not really satisfactory."
The Irish ME Trust is an information and support service for ME sufferers. See www.imet.ie for more details, or lo-call 1890 200 912.