# The seasons within: a theoretical perspective on photoperiodic entrainment and encoding

## TL;DR

This paper explores how organisms use internal clocks to adapt to seasonal changes in daylight.

## Contribution

It introduces conceptual phase oscillator models to explain photoperiodic entrainment and encoding.

## Key findings

- Photoperiod-dependent re-arrangements in the circadian system represent seasons internally.
- Phase oscillator models provide an intuitive framework for analyzing entrainment characteristics.
- Results are connected to more complex molecular models considering cellular and organism-specific details.

## Abstract

Circadian clocks are internal timing devices that have evolved as an adaption to the omnipresent natural 24 h rhythmicity of daylight intensity. Properties of the circadian system are photoperiod dependent. The phase of entrainment varies systematically with season. Plastic photoperiod-dependent re-arrangements in the mammalian circadian core pacemaker yield an internal representation of season. Output pathways of the circadian clock regulate photoperiodic responses such as flowering time in plants or hibernation in mammals. Here, we review the concepts of seasonal entrainment and photoperiodic encoding. We introduce conceptual phase oscillator models as their high level of abstraction, but, yet, intuitive interpretation of underlying parameters allows for a straightforward analysis of principles that determine entrainment characteristics. Results from this class of models are related and discussed in the context of more complex conceptual amplitude–phase oscillators as well as contextual molecular models that take into account organism, tissue, and cell-type-specific details.