The revolution of molecular biology in the early 1980s has revealed complex biochemical interactions underlying biological systems. To understand this
complex system, collaborations between biologists and mathematician have played important roles. The collaboration, however, is not always successful,
and it is even difficult to resume or maintain the collaboration. In this talk, I will share my experience of solving various puzzles related to circadian
(~24hr) rhythms, including 60 years old one, in collaboration with various experimental labs. Specifically, I will present how the combination of
mathematical modeling and experiments have successfully identified molecular mechanisms underlying robust circadian timekeeping and circadian
regulation of disease (e.g. Cancer). At the end of talk, I hope I can deliver a tip for using the combination of theory and experiment.