Supplemental lighting can be used in greenhouses to improve the growth, quality, and yield of greenhouse crops. However, the electricity cost associated with providing supplemental light can be high. The concave response of electron transport rate (ETR), an indicator of the overall rate of the photosynthetic light reactions, to photosynthetic photon flux density (PPFD) provides a valuable means for formulating supplemental lighting control strategies which reduce this energy expense by accounting for crop light use efficiency. The ETR response of ‘Little Gem’ lettuce (Lactuca sativa) was evaluated in a greenhouse over a 35-day production cycle, and the resulting information was used to propose energy-efficient supplemental lighting strategies. One means of improving crop supplemental light use efficiency is to provide the same quantity of supplemental light over a longer period of time at lower PPFDs, which is hypothesized to result in greater amounts of photosynthesis and increased plant growth. Greenhouse trials with ‘Little Gem’ lettuce demonstrate that this approach can be implemented in a greenhouse effectively using dimmable light-emitting diode lights, leading to improved growth and improvements in crop growth per Joule of energy expended on supplemental lighting. For a fixed photoperiod, simulations and numerical optimization demonstrate that the most efficient way to apply greenhouse supplemental light is to apply light in the most consistent possible manner over the course of a day. Simulations based on the ETR response of ‘Green Towers’ lettuce suggest that this approach can reduce energy use for supplemental lighting by as much as 10%. Because the response of ETR to PPFD can generally be described as an exponential rise to a maximum with only two parameters, it is possible to describe this response using only two measurements. This approach was tested and compared to data collected in a greenhouse and growth chamber for six annual bedding plant species. The two-point approximation generally somewhat over-estimates ETR but is reasonably accurate.