  The interplay between circadian and reward function is well understood in
animal models, and is of growing interest as an aetiological explanation in
psychopathologies. Circadian modulation of reward function has been
demonstrated in human behavioural data, but understanding at the neural level
is limited. In 2017, our group published results of a first step in addressing
this deficit, demonstrating a diurnal rhythm in fMRI-measured reward
activation. In 2018, Steel et al wrote a constructive critique of our findings,
and the aim of this paper is to outline how future research could improve on
our first proof-of-concept study. Key challenges include addressing divergent
and convergent validity (by addressing non-reward neural variation, and testing
for absence of variation in threat-related pathways), preregistration and power
analysis to protect against false positives, wider range of fMRI methods (to
directly test our post-hoc hypothesis of some form of reward prediction error,
and multiple phases of reward), the parallel collection of behavioural data
(particularly self-reported positive affect, and actigraphically measured
activity) to illuminate the nature of the reward activation across the day, and
some attempt to parse out circadian versus homeostatic/masking influences on
any observed diurnal rhythm in neural reward activation.
