The circadian variation of nitric oxide synthase (NOS) activity and cytosolic protein content in the cerebellum, brainstem, hypothalamus, hippocampus, and the remainder of the brain were studied in rats. Both NOS activity and cytosolic protein concentrations were the highest during the dark period and lowest in the light period. Hypothalamic NOS activity exhibited the most pronounced change in activity with time increasing by ~120% from mid-light to mid-dark.
Ayers, N. A., Kapás, L., & Krueger, J. M. (1996). Circadian variation of nitric oxide synthase activity and cytosolic protein levels in rat brain. Brain Research, 707(1), 127–130. https://doi.org/10.1016/0006-8993(95)01362-8