Lettuce growth is highly strongly affected by environmental conditions; therefore, efficient cultivation methods are required in plant factories to optimize lettuce growth. However, few studies have investigated the effects of the light cycle on lettuce growth under LED lights, although LEDs are the main light source currently used in plant factories. Moreover, few studies have investigated the effects of light cycles other than the24-h cycle on lettuce. In this study, we investigated the effects of different light cycles on lettuce growth under LED light. Lettuce (Lactuca sativa L. 'Greenwave', Takii & Co., Ltd.) was cultivated in a container-type growth chamber where air temperatures during the light and dark periods were maintained at 23 degrees C and 18 degrees C, respectively. Two separate experiments were conducted using 'Greenwave' lettuce: the effects of the light cycle (light period/dark period = 16 h/8 h, 16 h/4 h, 16 h/2 h) and photosynthetic photon flux (PPF) levels (110 and 170 mu mol m(-2) s(-1)) were investigated. A shorter dark period was found to promote lettuce growth, and the fresh weight of lettuce grown under a 16 h/2 h light/dark cycle was about 30% higher than that of lettuce grown under a 16 h/8 h cycle. Higher PPF promoted lettuce growth, but the efficiency of its production (fresh weight/electricity consumption) declined. To investigate the physiological reaction of lettuce to dark periods and PPF, experiments under higher PPF or other light cycles are required.