Interestingly, the protein pituitary adenylate cyclase-activating peptide-38 (PACAP-38), reported to have an altered expression in inter-bout CH patients, is also known to induce expression of PER1 and PER2, indicating a potential link, though there is yet to be a study to test this [10,11]. Here, PER1 is linked to cyclic hematopoiesis.