Balancing expectations: Adaptive flexibility in mammalian circadian rhythms

Circadian rhythms are internally generated oscillations of approximately 24 hours that synchronize with the environments day-night cycle, which drive and modulate countless behavioural and physiological processes. In this thesis we use a novel behavioural work-for-food paradigm which allows to study how changes in energy balance cause a change in the temporal niche of mice, making them adopt a day-active activity pattern. We show and discuss data supporting a functional role of circadian flexibility; diurnal activity patterns requiring less energy versus remaining night-active - for burrowing small mammals in temperate climate. We build on this by showing the rearrangements in temporal niche are associated with plasticity in the direct light response (photic masking) and explore differences between male and female mice. Further we show data that neither the adrenals nor the Paraventricular thalamic Nucleus are essential for circadian niche adaptation whereas the central circadian clock located in the suprachiasmatic nucleus remains of vital importance, despite itself not appearing to change phase. The rigidity of the SCN-timing might be linked to its role in measuring daylength and guiding seasonal rhythms. Which processes make behavioural and physiological rhythms obtain a different phase angle to the SCN during simulated food shortage remains largely elusive. The work in this thesis provides a solid scientific basis to re-address circadian flexibility and it’s relation to energy balance in future studies. Gaining more insights in circadian rhythm flexibility might solve the poorly understood mechanisms behind metabolic risks associated with human shift-work and how to cope with circadian disruptions.