The Period/Cryptochrome (PER/CRY) complex is a heterodimer that transcriptionally regulates circadian rhythm. When it is phosphorylated, it is transported to the nucleus and it inhibits a transcription factor that regulates the synthesis of its own subunits (negative feedback).
Research on the circadian molecular clock speaks of an accumulation of PER/CRY complex in the cytoplasm during the day phase (see review The circadian clock: pacemaker and tumour suppressor, Fu and Lee, 2003). Why does this complex not move to nucleus during the day? In other words how is the phosphorylation-dephosphorylation switch controlled?