Functional Evidence that NPY Promotes Sleep by Inhibiting NE Signaling (A–D) Tg(hsp:npy);dbh−/− and dbh−/− animals were less active (A and B) and slept more (C and D) than dbh+/− siblings during the day before and after HS. Tg(hsp:npy);dbh+/− animals were less active and slept more than dbh+/− siblings during the day after HS. NPY overexpression in Tg(hsp:npy);dbh−/− animals did not further decrease locomotor activity or increase sleep compared to dbh−/− siblings. Yellow bars indicate HS. Pre-HS and post-HS quantify data on day 5 before and after HS. (E–J) npy+/+, npy+/−, and npy−/− siblings were treated with either DMSO or prazosin. DMSO-treated npy−/− animals were more active (E and F) and slept less (H and I) than their DMSO-treated npy+/− and npy+/+ siblings during the day, but there was no difference at night (E, G, H and J). Prazosin decreased activity (E and F) and increased sleep (H and I) to a similar extent for npy−/−, npy+/−, and npy+/+ siblings during the day. Arrows indicate behavioral artifacts due to addition of water. Mean ± SEM for 2 (A–D) or 4 (E–J) experiments is shown. n.s., not significant; ∗p < 0.05; ∗∗p < 0.01; ∗∗∗∗p < 0.0001 by two-way ANOVA with Holm-Sidak test. See also Figures S6 and S7 and Table S2.