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The potential advantages of night-time heating of greenhouses by modern infrared (IR) radiative sources, instead of forced hot air, are examined experimentally and theoretically. Measurements of indoor and outdoor conditions during typical cold nights in central Greece were taken in an experimental greenhouse using either forced hot air or IR heating. A simple theoretical model that contains all the essential physics was developed and subsequently used in parametric studies. Experimental and simulation results confirmed that, with IR heating, inside air temperatures several degrees lower than the desired plant canopy temperature were sustained, and that this temperature difference increased proportionally to the nightly drop in outside temperature. The model estimated energy savings in the order of 45% to 50% using the IR sources currently available, and predicted significant further benefits from improvements in the radiative efficiency of the IR sources.