In recent years, an increasing number of grandparents have become their grandchildren's childminders. Now, a new study has found that while looking after grandchildren for one day per week can keep grandmothers mentally sharp, looking after them for five days a week can have a negative effect.

Australian researchers carried out mental sharpness tests on almost 200 women aged between 57 and 68.

They found that those who looked after their grandchildren one day a week performed the best in two of the three tests. However, those who looked after their grandchildren for five days a week did particularly bad on one of the tests, which assesses a person's mental processing speed and working memory.

The researchers were surprised about this finding. However, they noted that the more time grandmothers spent looking after their grandchildren, the more demanding they perceived their own children to be. As a result, mood may play a result in these findings, the researchers suggested.

"We know that older women who are socially engaged have better cognitive function and a lower risk of developing dementia later, but too much of a good thing just might be bad," commented Dr Margery Gass, executive director of the North American Menopause Society (NAMS), which published this study in its journal, Menopause.

The researchers have already begun further studies on this topic.

"Because grandmothering is such an important and common social role for postmenopausal women, we need to know more about its effects on their future health," Dr Gass added.