The paper “Mathematical Analysis of Light-sensitivity Related Challenges in Assessment of the Intrinsic Period of the Human Circadian Pacemaker“, co-authored by Imran Usmani, Derk-Jan Dijk (Surrey Sleep Research Centre), and Anne Skeldon, has been published in the Journal of Biological Rhythms (open access link at the journal). This paper is based on Imran’s PhD thesis at Surrey. Imran was jointly supervised by Anne and Derk-Jan and successfully defended his thesis in 2022. Mathematically, the paper shows that the gold-standard method for measuring the period of the human biological clock may not be the “true” period. This is quite a controversial idea but could explain observed differences between sighted and blind people. Perhaps not well appreciated by most people who can see is that those who are blind often find it hard to live on a 24 hour day. This is because our internal biological rhythms have a period that differs from 24 hours. We are entrained to 24 hours primarily by light that enters our eyes. The image below is Figure 1 from the paper.