Controls the night/day rhythm of melatonin production in the pineal gland. Catalyzes the N-acetylation of serotonin into N-acetylserotonin, the penultimate step in the synthesis of melatonin. 2-arylethylamine + acetyl-CoA = CoA + H(+) + N-acetyl-2-arylethylamine Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2. Monomer (By similarity). Interacts with several 14-3-3 proteins, including YWHAB, YWHAE, YWHAG and YWHAZ, preferentially when phosphorylated at Thr-29 (By similarity). Phosphorylation on Ser-203 also allows binding to YWHAZ, but with lower affinity (By similarity). The interaction with YWHAZ increases affinity for arylalkylamines and acetyl-CoA and protects the enzyme from dephosphorylation and proteasomal degradation. It may also prevent thiol-dependent inactivation (By similarity). Highly expressed at night in pinealocytes and in the retina. Expressed at very low levels in the hindbrain and midbrain. Exhibits night/day variations with an up to 150-fold increased expression at night. Up-regulation is due to a large degree to the release of norepinephrine from nerve terminals in the pineal gland and cAMP signaling pathway. Down-regulated by light-induced proteasomal degradation. In the retina, 10-fold increased expression at night. cAMP-dependent phosphorylation on both N-terminal Thr-29 and C-terminal Ser-203 regulates AANAT activity by promoting interaction with 14-3-3 proteins. Belongs to the acetyltransferase family. AANAT subfamily.