Acts as a transcriptional regulator (By similarity). Represses PER2 transcription through a recognition sequence in the promoter (PubMed:11427718, PubMed:15182670). Component of the circadian clock that may contribute to the rhythmic expression of PER2 gene in a light-dependent and time-of-day-dependent manner (PubMed:11427718, PubMed:15182670). Homodimer (By similarity). Binds DNA as a dimer (By similarity). Expression is regulated by light and circadian rhythms in the pineal gland (at protein level). Phosphorylated. Phosphorylated on Ser-182. Phosphorylation may require prime phosphorylation(s). Phosphorylation takes place at specific times of the day. Phosphorylation leads to its down-regulation through proteasome-dependent degradation. Belongs to the bZIP family. NFIL3 subfamily.