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-28 (By similarity). Phosphorylation on Ser-202 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). Highly expressed in pineal gland and in the photoreceptor outer segments in the retina. Expressed at about 100-fold lower levels in the pituitary gland and testis. Not detected in other tissues. Exhibits night/day variations with a 10-fold increased protein levels and activity at night in the pineal gland and a 5-fold increase in the retina. In both tissues, the mRNA levels remain constant. cAMP-dependent phosphorylation regulates AANAT activity by promoting interaction with 14-3-3 proteins. Phosphorylation levels exhibit night/day variations, with an increase during nighttime. Belongs to the acetyltransferase family. AANAT subfamily.