Menstrual cycle affects women’s reaction time, study finds

Women show their fastest reaction times during ovulation, but physical activity level has a stronger effect on brain function, new research suggests.

The study examined how menstrual cycle phases and physical activity influenced cognitive tasks that reflect real-world decision-making and reactions, such as timing, attention and response speed.

Researchers tracked 54 naturally menstruating women aged 18–40 through four stages of their menstrual cycles.

Participants showed their quickest reactions and made the fewest errors on the day of ovulation — when the ovaries release an egg ready for fertilisation.

Scientists from UCL found that while mental performance varied slightly across the cycle, far greater differences appeared between active and inactive participants.

Those who were inactive had reaction times around 70 milliseconds slower and made about three times more impulsive errors, regardless of cycle phase.

The findings could have implications for women’s sport, where reaction-time differences of around 20 milliseconds may determine whether an athlete avoids or sustains an injury such as concussion.

Earlier studies have also linked higher injury risk to certain menstrual cycle phases.

Dr Flaminia Ronca is lead author from UCL Surgery and Interventional Science and the Institute of Sport, Exercise & Health.

The researcher said: “This is the first time we’ve directly measured ovulation in this context and we found that cognitive performance was at its best during this phase, with participants reacting around 30 milliseconds faster compared with later in the cycle, during the mid-luteal phase before periods begin.

“At elite level, this could make the difference between sustaining a serious injury in a collision or not.

“But the really interesting finding for me is that the difference between those who were active and inactive was much greater — around 70 milliseconds — enough time for the brain to register a stimulus and initiate a voluntary reaction. That’s far more meaningful for everyday life.

“This shows the importance of incorporating some form of recreational physical activity into our lives. It doesn’t have to be intense or competitive to make a difference – and crucially, it’s something we can control.”

Participants were grouped by exercise level: inactive (no structured exercise), recreationally active (at least two hours a week), club-level competitors, and elite athletes competing nationally or internationally.

Each participant completed mood questionnaires and cognitive tests on the first day of menstruation, two days after it ended (late follicular phase), the day ovulation was detected, and between ovulation and menstruation (mid-luteal phase).

Reaction times were slower during the mid-luteal phase, likely due to higher progesterone levels — a hormone known to slow brain processing.

However, accuracy did not decline, suggesting slower responses do not necessarily reduce performance quality.

More errors occurred in the late follicular phase, just after periods ended, though the reason remains unclear.

While 55 per cent of participants believed menstrual symptoms affected their performance, the researchers found no evidence to support this.

Reaction times during menstruation were actually faster than during the mid-luteal phase.

“It’s great to see that while participants assumed they were performing worse during menstruation, the findings don’t show any decline in cognition,” said Evelyn Watson, study author from UCL Surgery and Interventional Science and the Institute of Sport, Exercise & Health.

“If anything, cognitive performance peaked during ovulation.

“This is a positive outcome that we hope can help develop a new narrative in female health and performance.”

Dr Ronca added: “Working exercise into our day doesn’t need to be difficult.

“Some of our previous studies have shown that 15 minutes of moderate activity is enough to boost mood and cognitive performance — that’s equivalent to a brisk walk around the block or cycling to the shops.”

The 70-millisecond gap between active and inactive groups could be enough to determine whether someone regains balance after tripping, underlining how physical activity supports brain function beyond sport.