Women may be more affected by shiftwork than men.

By Dani Cooper. 

Women are more likely to be affected by jobs involving night shiftwork than men, according to a new study.

The finding comes from a paper published today in the Proceedings of the National Academy of Science journal that details for the first time how changes in sleep-wake cycles and circadian rhythm differently influence brain function in men and women.

To track these differences, lead author Professor Derk-Jan Dijk, from the University of Surrey, and colleagues from the UK and Singapore rescheduled the sleep-wake cycles of 16 men and 18 women to a 28-hour day, which involved going to bed and waking four hours later each day.

For the 10-day experiment, all time cues and external light were removed from the laboratory and low-light conditions maintained during “waking hours”.

Dr Dijk, director of the Surrey Sleep Research Centre, said sleep was regulated by two systems — the body’s sleep-wake cycle and the circadian biological clock.

The sleep-wake cycle helps people maintain enough sleep throughout the night to balance against the time they are awake, while the circadian clock regulates the timing of periods of sleepiness and wakefulness.

The circadian clock adjusts according to environmental time and light levels — in general adults’ strongest sleep drive is between 2am and 4am.

28-hour day tests the sleep cycle.

Dr Dijk said the creation of a 28-hour day disrupted the sleep-wake cycle and made it easier to determine the impact of the body — or brain — clock on performance.

“We normally sleep at night and are awake during the day, and this makes it difficult to assess the separate contribution of ‘internal time of day’ as determined by the brain clock and the duration of wakefulness,” he said.

“We can assesses the separate contributions of these two factors by scheduling the sleep-wake cycle to a 28-hour day. The brain clock ticks at approximately 24.2 hours and cannot keep up with this 28-hour day.

“When we do this for a week or so, sleep and wakefulness have been scheduled at … the internal brain time and we can then estimate the contribution of time awake and brain time to performance.”

Participants undertook a series of cognitive tasks and self-assessments every three hours while they were awake.

The activities included rankings of sleepiness, mood and effort required to complete tasks and tests to measure attention, accuracy and motor control.

Dr Dijk said the study showed in women the performance on certain tasks was more impaired by “being awake at the wrong time of day” than in men.

“Extrapolation of these findings to the real world implies that women may be more affected by shiftwork than men,” he said.

The researchers also suggest this difference “may in part reflect social factors such as family and childcare responsibilities that lead women to work longer hours and to sleep less on days off than men”.

Women under-represented in sleep studies.

Dr Dijk said the study was the first to investigate sex differences in circadian regulation of performance while awake.

He said women were generally under-represented in sleep studies because of concerns that any variability between women and men was due to “variations in hormones related to the menstrual cycle”.

“But, if we are interested in the effects of circadian rhythms and sleep on brain function in humans there is of course no good reason to limit ourselves to only men,” he said.

This post originally appeared on ABC News.