Response to 'Comment on 'Domestic light at night and breast cancer risk: a prospective analysis of 105 000 UK women in the Generations Study".

Dear Editor,

We thank Drs Kyba and Spitschan for their thoughtful comments.[1] We agree that the responses we received[2] from our study participants on extent of light at night in bedrooms are a complex reflection of actual visual ability, subjective judgement of visual ability, and recall and generalisation of this. It may indeed be possible to develop more-valid questions, especially, as Drs Kyba and Spitschan say, if these can be evaluated against measured light in the bedroom. However, it will not be simple since, for instance, actual light at night will vary by season and according to the weather.