[rcyeh]

Interesting, I initially thought this factor was probably negligible, because the cohort was aged "62.8 ± 7.8 years" (either before or after the 7-year observation period). I still think it's small, but perhaps not negligible.

I estimate that some 2-4% of the group could be heavily involved in caring for grandchildren, sorting themselves into the irregular category. It's well-known that school-aged children pass flu to grandparents [2], and then grandparents die, just in time for the 7-year post-birth observation window. The absolute death rate due to flu is 10^-4 to 10^-3 per year, which would be visible on the paper's mortality time course chart.

Estimation details:

* some 20% are grandparents (and very few are parents) of young children [0] * 20-50% of grandparents care for grandchildren regularly [1]

So maybe 4-10% of the cohort as an upper bound. If the birth rate is 12 per 1000 per year, and babies cause sleep disruption for about two years to two persons, then that's about 4% also, but perhaps mostly not the same age group.

[0]: https://assets.publishing.service.gov.uk/media/5eeb975b86650...

[1]: https://www.ageuk.org.uk/latest-news/articles/2017/september...

[2]: https://www.sciencedirect.com/science/article/abs/pii/S00255...

Going to sleep now.