Expression of cryΔ5'.Scer\UAS.cDa.T:Ivir\HA1, under the control of Scer\GAL4tim.PE in flies exposed to constant darkness results in the flies exhibiting a significant increase in period length. The free-running period for these flies is 1.5-2.7 hours longer than for controls. Expression of cryΔ5'.Scer\UAS.cDa.T:Ivir\HA1, under the control of Scer\GAL4P2.4.Pdf or Scer\GAL41118 in the small and large LNVs is sufficient to increase the free-running period compared to controls.