They will be fine for 24 hours. No need to worry. Also, if you don’t want the big light on for a day, you can always supplement a compact florescent bulb in the grow room. This won’t add any weight and is not nearly enough energy to grow anything worth while. But will effectively break up up the dark cycle and stop a plant from blooming if done correctly. Look up gas lantern routine growing. It talks about this method of breaking up the dark cycle to avoid blooming.