PACAP-null mice re-entrain to a phase-advancing lighting schedule, but at a significantly slower rate. Representative double-plotted behavioral actograms for two WT (PACAP +/+) and two PACAP-null mice (PACAP −/−) show patterns of re-entrainment to a new lighting schedule either advanced (top) or delayed (middle) by 6 h. Shaded areas represent darkness. When mean rates are evaluated daily (graph, bottom), significantly fewer PACAP −/− mice (N=21) than PACAP +/+ (N=31) have re-entrained to a 6-h phase advance on day ≤1 and ~45% still are re-entraining at ≥ 4 days, when all WT mice have completed the shift (p < .05, X2). The two genotypes exhibit similar rates of re-entrainment to 6-h phase delays.