Involved in the generation of biological rhythms. The biological cycle depends on the rhythmic formation and nuclear localization of the tim-per complex. Light induces the degradation of tim, which promotes elimination of per. Nuclear activity of the heterodimer coordinatively regulates per and tim transcription negative feedback loop. Behaves as a negative element in circadian transcriptional loop. Does not appear to bind DNA, suggesting indirect transcriptional inhibition (By similarity). Expression exhibits prominent circadian variation in adult heads and in particular in the photoreceptor nuclei. Forms a heterodimer with timeless (TIM); the complex then translocates into the nucleus. Nuclear at specific periods of the day. Interaction with TIM is required for nuclear localization (By similarity). Phosphorylated with a circadian rhythmicity.