Tracie Scott-Jensen knew something had to change. Over her 10 years working regular night shift rotations as a registered nurse, she had been plagued with illness and exhaustion.
Living on three hours sleep a day, she had endured stomach cramps, constipation, urinary tract infections and weight gain. Fatigue had even caused her to crash into her supervisor's parked car and fall asleep at traffic lights. But this was nothing compared to her fear that her tiredness would eventually imperil one of her patients.
''I can't explain the fear that you are incapable and can't look after your patients. I felt I was a good nurse during the morning shift and afternoon shift but, on the night shift, I just wasn't good … because I was tired.''
Scott-Jensen's body just could not adjust to night work. She has a condition that has recently been labelled shift-work sleep disorder.
''I had to get prescribed sleeping tablets by my doctor so I could do the night shift,'' says Scott-Jensen, whose nightmare ended only after her doctor certified that she could not work at night for health reasons.
She chose to quit her job in a hospital near her home in Tennyson in northern Victoria because of a fear of having to return to night work, choosing instead to travel 2½ hours to Melbourne.
Researchers say there are many questions about how shift work affects the body. About 1.4 million Australians are shift workers and demand for a 24-hour workforce is growing. Yet studies show the 15 per cent of the Australian workforce who work regular night shifts may be putting their health at risk.
Since 2007, when the World Health Organisation listed night shift as a probable carcinogen, a range of studies have associated working at night with health issues from diabetes and obesity to heart disease and cancer.
Associate Professor Shantha Rajaratnam, from Monash University's faculty of medicine, nursing and health science, says it is only in the past five years that scientists have really begun to understand how tampering with the body's natural sleeping patterns can affect us physiologically.
''We didn't realise before that disrupted circadian rhythms have an impact on the cardiovascular system, the reproductive cycle and on the digestive system,'' Rajaratnam says.
The circadian clock, the body's biological timer, controls the timing of hormone secretion, body temperature and feelings of alertness and sleepiness. Thousands of years of evolution have fine-tuned these circadian rhythms to rise and fall in concert with the rising and setting sun - in humans, this is an average 24-hour and 22-minute cycle.
The problems with shift work are caused by a mismatch between the body clock and the sleep wake cycle.
''The body clock is adapted to do a particular light-dark cycle which matches the light-dark cycle in which you are currently living,'' Rajaratnam says. ''When you voluntarily shift your sleep and wake times to a new time zone, your body clock often fails to adapt because it's getting competing information.''
Studies repeatedly tell us shift workers are more likely to lead an unhealthy lifestyle. They are more likely to smoke, exercise less and eat food higher in fats and sugars.
A study published last month on the British Medical Journal's website - in which researchers looked at 34 studies that involved more than 2 million people - found night workers were 41 per cent more likely to have a coronary event such as stroke or heart attack.
It called for screening programs to identify risk factors in shift workers.
''Lifestyle factors no doubt contribute,'' Rajaratnam, who is also the president of the Australasian Sleep Association, says. ''But I think laboratory studies over the last few years suggest there are internal physiological factors that also contribute to the risks.''
He says night shift workers do not usually sleep well during the day because their body clock is trying to keep them awake.
This is because the circadian clock controls the timing of melatonin, a hormone that induces sleep. Exposure to light at night inhibits the production of melatonin, which has been linked to an increased risk of cancer.
Rajaratnam, who says even standard room lighting is enough to have an impact on melatonin production, is leading a pioneering study that will look at how blue light can be used in workplaces to keep shift workers alert.
Blue light has a high colour temperature and targets the biological structures that affect the body clock.
One of the circadian clock's main drivers is light, so adapting to a new time zone involves completely avoiding light at some times of the day and being exposed to light at others.
He says shift workers could potentially be given light therapy to help adjust to a night schedule, so they would be more efficient at work and better able to sleep during the day.
Darren O'Brien, a consultant at the Woolcock Institute of Medical Research in Sydney, says the importance of sleep is undervalued.
''If you consider that we spend a third of our lives sleeping, it [night shift] has to be having an impact on our health,'' he says. ''Sleep is an active state. Even though we are unconscious, there's a lot going on. It's important for memory consolidation, learning, retention of knowledge, concentration and performance.''
When we miss out on sleep in more than one 24-hour cycle we accumulate a sleep debt and it is a myth that we can ''catch up'' on missed sleep with a long lie in on a day off, he says.
Mathematically you might think you have compensated but ''you usually don't get the kind of restorative sleep that you need''.
Margaret Cusak thought it was normal for her to be tired. A registered nurse with 25 years' experience on the night wards, she loved working night shift and thought she was coping well. That was until she watched a video of herself coaching her teenage girls at basketball.
''I got such a shock. I moved as if I'd been out on a hard night's drinking and it suddenly hit me how dangerous I am,'' she says.
''I thought … 'I'm driving my kids around in this state'. That's when it hit me what night duty was doing to me without me even being aware of it.''
Cusak has now returned to day work and feels much better. ''In a night shift, you feel permanently jet lagged but that was a way of life for me.
''You never get nearly the amount of sleep you need working nights.''
Shift work can be harder for women, who typically carry the burden of responsibilities in terms of childcare, and have less time for sleep, says Fiona Baker, a senior research scientist at the human sleep research program at the not-for-profit independent research centre, SRI International in California.
Woman are known to sleep more deeply than men, she says, but women report getting less sleep than male shift workers, report more sleeping difficulties and more fatigue.
Baker says women could also be uniquely affected by night shifts because of the relationship between the menstrual cycle and the circadian clock. Receptors for the reproductive hormones oestrogen and progesterone found on the circadian clock also suggest a link between the circadian rhythms and the reproductive system.
About half of female shift workers report irregularities in their periods when they work night shift as well as increased symptoms of PMS.
Evidence from questionnaire-based studies suggests women who work long-term night shift may also have delayed ovulation and find it harder to get pregnant, but scientists say there is much more to learn.
Weight gain can be another side effect of working nights. Disruption to the body's master clock affects the secretion of digestive hormones. The body's ability to process sugar is also impaired by circadian misalignment. And laboratory studies show the changes occur within days of altering the sleep wake cycle.
These changes are compounded by sleep deprivation, which can increase cravings for high-fat, high-sugar foods.
Marianna Henry, 48, a hospital scientist, says that for her putting on weight was the most traumatic aspect of working nights.
''I've put on 15 kilos. It was very unlike me because I've always been … thin but you have to eat at night because you can't stay awake.''
Despite working regular night shifts for 18 years, she still cannot adapt to a daytime sleeping routine.
''I only ever seem to sleep four hours before I wake up, even on my days off,'' she says.
Sandra West, associate professor of clinical nursing at the Sydney Nursing School, says medical diagnoses of shift-work sleep disorders tend to ''make the shift worker the problem''.
Organisational changes, such as better rosters, provision of healthy food and educating night workers about how to ensure they sleep longer in the day are some ways the lives of our night force could be improved, she says.