Nicotinic acetylcholine receptors are a major class of ligand-gated ion channels in insect nervous system. Despite being well studied as insecticide targets, their roles in insect behaviours are still poorly understood. This study identified one of the ten receptor subunits, Dα1, as a modulator of sleep and circadian rhythm in Drosophila melanogaster as well as courtship behaviour. Using ends-out gene targeting, a genomic deletion of Dα1 was generated. Null mutant flies showed decreased night sleep which was due to shorter sleep episodes. Without a cycling light-dark condition, a severe loss of both sleep and circadian rhythm was observed. Additionally, courtship behaviour was also greatly affected in mutant flies. Unsurprisingly, their lifespan was significiant reduced to almost half that of control flies. Pan-neuronal GAL4 driven expression of the wildtype allele in the null background was sufficient to revert most, but not all, of these phenotypic changes. Even though mutations in Dα1 clearly confer a high level of resistance to neonicotinoids, a main class of commercial insecticides, fitness costs associated with a complete loss of Dα1 might explain the low observed frequency of such mutations in the targeted pest specieis.