The study is the first to show that shifted sleep-wake cycles affect men and women’s ability to function differently.
The study – from the Surrey Sleep Research Centre (SSRC) at the University of Surrey, UK, and published in the Proceedings of the National Academy of Sciences – is the first to show that shifted sleep-wake cycles affect men and women’s ability to function differently, revealing key differences in cognitive performance and changes of mood.
First author Dr. Nayantara Santhi, a research fellow at the SSRC, says:
“Extrapolation of these results would suggest that women may be more affected by night shift work than men.”
The study could have significant implications for women who work night shifts in professions such as nursing and the police, suggest the authors.
Our body clock, or circadian rhythm, regulates the daily cycles of our bodies as we transition from day to night, and from wakefulness to sleep. A “master clock” in the brain coordinates the clocks of various different processes – for example, hormone production, metabolism and blood pressure – so they are in sync.
There is evidence that some circadian features – such as the frequency of the clock and the size of highs and lows in production of the sleep hormone melatonin – differ between men and women, but the authors note they are the first to see if these differences extend into mental function.
Circadian rhythm changes affect brain differently in men and women
For the study, the researchers put 16 male and 18 female volunteers on 28-hour days in a controlled sleep lab at the SSRC. This meant that they experienced day-night changes on a 28-hour pattern instead of the 24-hour pattern of their inbuilt circadian rhythm.
As the days went on, the participants began to sleep out of sync with their internal clock – similar to what happens when working shifts or due to jet lag, for example.
Every 3 hours during the wakeful part of their 28-hour day, the participants underwent a range of objective tests of their performance – such as attention, motor control and working memory. They also completed subjective tests – self-assessments of sleepiness, mood and effort.
The researchers also took continuous readings (electroencephalograms, or EEGs) of electrical activity in the participants’ brains as they slept.
The results showed that for both men and women, the subjective measures – the self-assessments – were more sensitive to the effects of time awake and circadian rhythm than many of the objective measures of performance.
But the more crucial finding was that the effect on the objective measures of performance was stronger in the women’s results than in the men’s; they showed the women performed less well during the early morning, which would be around the time a night shift worker comes off a night shift.
Senior author Derk-Jan Dijk, professor of sleep and physiology at the University of Surrey, concludes:
“These results show that in both men and women circadian rhythmicity affects brain function and that these effects differ between the sexes in a quantitative manner for some measures of brain function.”
He also points out that the study highlights the importance of including male and female subjects in this kind of research and of using a range of subjective and objective measures of brain function.