Vasoactive intestinal polypeptide (VIP) signaling regulates the amplitude, period, and phase of SCN neuronal rhythms. A: amplitude of SCN neuronal rhythms is significantly reduced in VIP/PHI-/- mice, compared with wildtype animals (Tukey test). Data are means ± SE; numbers in parentheses indicate number of rhythmic animals; * = P < 0.05. B: number of arrhythmic SCN neurons increases in VIP/PHI+/- and VIP/PHI-/- mice compared with wild ype, while the remaining rhythmic cells show altered free running periods. C: time of peak firing in rhythmic SCN neurons from VIP/PHI+/+ and VIP/PHI+/- mice tends to occur around the mid projected day, whereas oscillating cells from VIP/PHI-/- mice peak throughout the late day and projected night.