A genetic approach was attempted to identify the gene responsible for nyctinastic movement in legume. Seeds of the model legume Lotus japonicus were treated with ethylmethane sulfonate and screening of 40,000 M2 seeds led to the isolation of one mutant named sleepless. sleepless is incapable of closing its leaflets towards the adaxial side at night. The pulvini at the leaflet base were found to be replaced with petiole-like structure in sleepless. Wild-type pulvini comprise many compressed cells, whereas the corresponding region in sleepless is made up of roundish cells in the cortical parenchyma and highly elongated cells in the epidermis, particularly in the leaf-length direction. Based on the results of histological examination, I propose a possible model of a developmental pathway leading to nyctinastic movement.