•Cos lettuce plants were grown under irradiation patterns in which spectral photon flux density distribution changes over time.•Growth was promoted by shifting the irradiation times of blue and red lights.•Diurnal PPFD change contributed to the growth promoting effects.•Blue and red light monochromatic irradiation contributed to elongation of leaves. This study examined the growth and morphology of cos lettuce (Lactuca sativa L.) to ascertain the effects of irradiation patterns with combinations of blue and red LED light, with spectral photon flux density changing with a period of 24h. In experiment 1, plants were grown under irradiation patterns in which plants were irradiated with blue and red LED lights at a PPFD of 90μmolm−2s−1 for 14h per day respectively. The red LED light irradiation starting time was simultaneous to or delayed 1, 4, or 7h from the blue LED light irradiation starting time. Results showed that the shoot fresh weight of plants grown under irradiation patterns with red LED light irradiation being delayed 4 or 7h from blue LED light irradiation was significantly greater than that under irradiation patterns with blue and red LED light irradiation starting simultaneously. This result showed that the growth of plants can be promoted merely by temporally shifting the irradiation hours of blue and red LED lights. We then conducted experiments to assess the mechanism of the observed growth-promoting effect from viewpoints of the diurnal PPFD change and duration of blue and red light monochromatic irradiation. The results of these experiments revealed that the diurnal PPFD change contributed to the growth-promoting effect and that blue and/or red light monochromatic irradiation increased the total leaf area of plants.