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). 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.