Mechanisms of non-photic entrainment

This chapter reviews how non-photic cues like temperature, food, anxiety and induced activity lead to circadian rhythm entrainment in mammals. Generally speaking, when dealing with entrainment by different photic and non-photic zeitgebers, we are only able to show that a certain type of stimulus can or cannot entrain the circadian system, therefore, the comparison is difficult to be quantified for the purpose of interpretation. For example, the effect of temperature as a zeitgeber could be different when used alone or when used in anti-phase with the light-dark cycle. Timed feeding and anxiety can also cause non-photic entrainment of circadian rhythms, with a potentially strong selective benefits for prey species like mice and rats. Although significant work has been done on non-photic entrainment, there are still many open questions to date. Here we will describe different aspects of non-photic entrainment of the circadian system in mammals, with focus on certain open questions especially when nocturnal and diurnal animals are compared.