"After a month of having such little sleep, a person is not in a fit state to look after a baby," says Dr Reyner. Not good if you are attempting to feed, clothe, bathe and care for a vulnerable child, around the clock. Their memory will be severely impaired and they will have trouble initiating behaviour."You may also feel apathy, lack emotion, become distracted by irrelevant stimuli and suffer from inflexible thinking as well as impaired communication skills. Women who have severe sleep deprivation tend to get postnatal depression. "There will be severe concentration problems and difficulty remembering things," says Dr Reyner.

"There will be general mood disturbances, which could lead to very severe mood swings. One in six car crashes that result in injury or death on major roads is caused by drivers falling asleep at the wheel.After a month, the effects will become much more pronounced. "I would argue that after one night of just three-and-a-half hours' sleep you should not be driving," says Dr Reyner. Anyone who has to survive on three-and-a-half hours a night will soon feel serious effects. After a week you will not only feel very tired, but also have very poor concentration and have difficulty in planning or thinking for yourself Your moods will also be disturbed. So too is the anxiety working mothers feel about getting a good night's sleep.It's not just mums.

One woman in five and one man in 10 reports being "abnormally" tired. And as any of them will testify, if you lie awake worrying that there are only five, or four, or three hours until the alarm goes off, there is no chance of getting to sleep.All this can be highly dangerous, says Dr Louise Reyner of the sleep research centre at Loughborough University. Baby monitors that send parents leaping out of bed at the slightest gurgle and reluctance to put babies in a cot or leave them to cry are blamed. But now it seems women are getting even less sleep than Margaret Thatcher's famed four hours.