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-31 (By similarity). Phosphorylation on Ser-205 also allows binding to YWHAZ, but with lower affinity (By similarity). The interaction with YWHAZ considerably 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). High levels in pineal gland and retina. Exhibits night/day variations with an increased expression at night. Higher levels in pineal gland in early morning than in early afternoon (at protein level). cAMP-dependent phosphorylation on both N-terminal Thr-31 and C-terminal Ser-205 regulates AANAT activity by promoting interaction with 14-3-3 proteins. Belongs to the acetyltransferase family. AANAT subfamily.