Day–night differences in neural activation in histaminergic and serotonergic areas with putative projections to the cerebrospinal fluid in a diurnal brain

Highlights • Fos expression in brain sleep/wake areas predict a species temporal activity pattern. • Serotonergic neurons show daily rhythms of Fos expression in the diurnal grass rat. • Fos expression in the grass rat suprammamillary nucleus peaks at dusk and dawn. • Neurons of the grass rat tuberomammillary nuclei show rhythmic Fos expression. • Rhythmic serotonergic neurons of grass rats may have access to the ventricular system.