Neuropeptide PDF is the main transmitter regulating circadian locomotor rhythms. Required to maintain behavioral rhythms under constant conditions by coordinating pacemaker interactions in the circadian system. Ectopic expression induces long periods, while its absence leads to short periods. Acts on small and large ventral lateral neurons to control sleep and regulates the state transition from sleep to wake (PubMed:19038223, PubMed:19230663). Predominantly expressed in adult head. Expressed at higher level in males than in females. In adult brain, it is specifically expressed in the ventral lateral neurons (LNvs) as well as in 2-4 tritocerebral cells and 4-6 abdominal cells. Expressed at constant level, without strong circadian variations. Its amount nevertheless oscillates along day and night cycles in the termini of dorsally projected axons of LNvs. Increases total sleep time, decreases sleep latency, leads to loss of morning activity peak, and a failure to wake up in advance of light-on anticipation. Belongs to the arthropod PDH family.