As part of the circadian clock, "timeless" is essential for entrainment to light-dark (LD) cycles. The typical period length of free-running "Drosophila" is 23.9 hours, requiring adaptations to the 24-hour environmental cycle. Adaptation first begins with exposure to light. This process leads to the rapid degradation of the TIM protein, allowing organisms to entrain at dawn to environmental cycles. In light-dark cycles, TIM protein level decreases rapidly in late night/early morning, followed by the similar but more gradual changes in PER protein level. TIM degradation is independent of "per" and its protein, and releases PER from the PER/TIM complex. In some cell types, the photoreceptor protein cryptochrome (CRY) physically associates with TIM and helps regulate light-dependent degradation. CRY is activated by blue light, which binds to TIM and tags it for degradation. This ends the PER/TIM repression of the CLK/CYC-mediated transcription of "per" and "tim" genes, allowing "per" and "tim" mRNA to be produced to restart the cycle.