When people start to doze in meetings "they are in big trouble" if they tell themselves to not fall asleep, says organisational psychologist Ross Gilmour.

"What they need to be telling themselves is, 'I stay awake, I concentrate, I stay awake."'

The instruction "I must not fall asleep" actually creates the image of falling asleep in the mind, just like telling children not to touch the television increases the chances they will.

Studies show that approximately 30 per cent of people working "normal" hours in New Zealand experience sleep problems in a year and about 10 per cent describe the experience as very distressing. The figures for shift workers are significantly higher (50 per cent plus).

The effects of poor sleep on safety, quality of performance and health can be significant, Gilmour says; it's something employers may not think to ask their daytime workers.

Common symptoms are: making mistakes, an inability to concentrate and grumpy behaviour.

"People will go 'oh well, they're a bit stressed' but frequently that can be because they're not sleeping well. Many times when you get stressed, that interferes with sleep and you can get a vicious cycle going."

Gilmour identifies two causes - initial and ongoing.

Initial causes such as depression, grief, ageing, boredom, trying to lose weight, pain and discomfort after an injury, an unusually stressful project or shift work may set up irregular sleep patterns.

Ongoing causes have more to do with the messages people give themselves. For instance, if a one-off project has been particularly stressful, a person may start waking at 2am. That is the initial cause.

But when the project is finished and the sleep interruptions continue, the brain is getting an unconscious instruction: "I wake at 2am."

Gilmour recommends giving conscious messages such as "I soon get back to sleeping well" or "my good sleep patterns are returning", followed by "I sleep right through the night until just before my chosen time to wake".

"The powerful part of the brain is working quite differently from the logical, conscious part. And it's used to instructions. When we go to pick up a drink, we don't have to tell our arm: 'Move towards it, close fingers, lift it up."'

Shift work, and particularly night shifts, significantly increase the incidence of disturbed sleep patterns and resulting fatigue.

Driving after a long shift is risky as concentration levels are low. Studies that compare the effects of moderate sleep deprivation (17-19 hours) and the influence of the circadian biological clock with those of legal amounts of alcohol find they are similar when it comes to driving.

For example, reaction time, attention, judgment, control and driving ability are impaired and there is an increased likelihood of a car crash or near-miss after extended shifts.

Drivers, aviation staff, miners, police officers, nurses, doctors, printers and building and construction and agriculture workers are often required to work shifts.

Registered nurse Marie Habowska has been in practice since 1982 and works three days a week in an acute setting and advises the NZNO.

But between 1997 and 2002, Habowska worked rotating eight and 12-hour shifts in an intensive care unit. She has many colleagues her age and older who are still working that way.

Habowska "hit a wall" in her 40s and resigned because of the continued requirement to do night shift.

That particular unit needed 14 staff day and night. To minimise the disruption she did two day shifts and one night shift. When working three nights in a row, "the quality of my wakefulness was appalling. I felt I could only just do my job. Your life is dominated by when night shift is coming up and 'How much sleep have I had?'."

She says her concentration was diminished and she had to double-check everything. She drank coffee to stay awake and was grateful to work in an area with bright lights.

After the shift she would drive home - an hour away - with the windows down, "wind blowing your face to keep awake".

"I didn't care how cold it was. I turned the heater off in the car or put it on cold and turned the radio up."

Sometimes she slept well during the day, other times she didn't. She would rush home to bed and take the phone off the hook - doing as little as she could to avoid becoming too wakeful.

Gilmour says that shift work compounds existing sleep problems, although people can train themselves to sleep through all sorts of things.He trained himself to sleep on helicopters during work assignments overseas.

"Where shift workers need to learn special techniques is when they go past two nights in a row. Once they extend that to a week of nights, unless they are good at sleeping during the day, they will have problems."

And maintaining the same eating patterns is advisable as the body is not interested in digesting between 2am and 4am.

"If a shift worker eats big in the middle of the night it's just going to sit there."

So the idea is to eat the same regardless of whether it is the beginning or end of the working day.

Under the Health and Safety in Employment Act 1992, employers have a duty to prevent harm occurring to employees while at work. Obligations include the effects arising from excessive work hours or insufficient rest periods.

The Department of Labour enforces health and safety law and responsibilities. The department provides resources, codes of practice and has a team of labour inspectors to investigate breaches of the law. In 2007 the department released a guide for employers on how to manage shift work to minimise workplace fatigue.

"The guide's major recommendation is that night work between midnight and 6am is limited to three nights after which workers must have two full nights off for sleep. There are also good arguments for more breaks for night workers."

The New Zealand Council of Trade Unions recognises stress and fatigue as hazards which can have serious physical and/or mental consequences for employees.

"Occupational diseases, including serious consequences of stress and fatigue, are potential hazards to be addressed by prevention strategies."

Massey University's Sleep/Wake Research Centre has aided a range of workers such as rail, marine, aviation, medical and trucking personnel.

"We often work with an organisation that has identified a particular problem or are seeking to make a change to their working hours and wish to have scientific guidance on the best way to proceed," says associate director Leigh Signal.

The centre worked with air traffic controllers during an investigation into the effectiveness of planned naps on night shift for maintaining worker performance.

"We were able to demonstrate that a short nap during a scheduled break reduced signs of sleepiness and helped individuals maintain their performance through until the end of the night shift."