A month's bad sleep can lead to poor mental health as far as a year later, new research that tracked thousands of New Zealanders over six years has found.
Studies have already shown how more than a third of Kiwis don't get the recommended seven to nine hours of shut-eye each night – and there's more than enough evidence linking poor sleep to problems ranging from heart disease and diabetes to depression and anxiety.
But less was known about whether psychological problems came as a result of shorter sleep - or the other way around – over long periods of time.
"It's an important question, as understanding the direction of effects can help with informing policy makers and public health professionals on where to intervene," said the new study's leader, Dr Mathew Marques of Australia's La Trobe University.
He and fellow Australian and Kiwi researchers found a way to shed more light on the issue through the University of Auckland-run New Zealand Attitudes and Values Study (NZAVS) - a longitudinal programme tracking thousands of Kiwis over 20-years - and clever new mathematical models.
Once every year between 2013 and 2018, large pools of Kiwi participants were asked to report how many hours of sleep they'd received each night the previous month - as well as where they sat on a scale measuring psychological distress.
To see how this data had changed over time, the team used special models that were able to predict how sleep duration affected psychological health, over a year in time - and all while controlling for individual differences in variables.
"This is pretty neat, because you or I may on average sleep a different number of hours, and the model is able to account for that, and predict the effect of change from the average level," Marques said.
Eventually, they found some evidence to suggest that poorer sleep over a short period of time - such as a month - could indeed lead to mental health problems a year later.
And, interestingly, it wasn't psychological distress that drove that lack of sleep in the first place - meaning there wasn't necessarily a vicious cycle at play.
"It does not appear that the effect goes from psychological distress to sleep."
Elsewhere, the study found that, without accounting for individual differences among participants, higher levels of sleep led to lower psychological distress a year on - and lower levels of psychological distress also led to higher levels of sleep a year later.
Marques said studies had shown that Māori and Pacific people, along with poorer people, suffered from greater sleep problems, or simply not getting enough of it.
There were many factors for this, ranging from poorer health generally, to having a higher likelihood of doing shift or night work.
"Importantly, these groups also persistently exhibit higher rates of psychological distress and mental health problems," Marques said.
"Addressing factors that lead to shorter sleep among these groups may thus help improve their psychological wellbeing over time."
Last year, University of Auckland researcher Carol Lee - also a co-author on the new paper - used NZAVS data to find that nearly 40 per cent of Kiwis were spending fewer than seven hours on the pillow each night – short of the optimal seven to nine hours.
The results of her study showed a total 58 per cent were getting optimal sleep, while 37 per cent were getting less than seven hours, and 4.9 per cent were getting more than nine hours.
As well, that research drew a line between shorter sleep and poorer mental wellbeing.
Along with psychological problems, tiredness and lack of sleep remained a factor in hundreds of road crashes each year, along with hundreds of millions of dollars in related costs.
Road crash researchers have pointed to people driving long distances without pulling over, and also to workers clocking up too much overtime before climbing into the driver's seat.
More than a quarter of Kiwis work more than 40 hours a week - and one previous study has indicated as many as one in 10 drivers falls asleep at the wheel.
Getting enough sleep
• The National Sleep Foundation recommends seven to nine hours of sleep a night for adults up to the age of 65, and seven to eight hours for those over 65. Some people naturally sleep slightly more or slightly less than these recommended hours.
• People can establish regular sleep pattern by going to bed and getting up at around the same time each day, including at weekends.
• It's important to relax and unwind before bed. That means spending the last hour awake doing calming activities like reading, having a shower or bath or doing mindfulness activities.
• If still awake after 20 to 30 minutes of trying to get to sleep, it's best not to stay in bed. Try sitting another room reading for a short time. Sleeping pills should be avoided unless recommended by a doctor or pharmacist, and be only used occasionally, or over a short term.
Source: Ministry of Health