Every plant on Earth recognizes that a change in light lasting longer than 4 minutes is probably not just a flicker of the light source (e.g. fluctuations due to cloud cover) and therefore increases its photosynthesis efficiency. If the plant is incorrect in its analysis, it runs the risk of burning to death by increasing its photosynthesis efficiency as the sun would consequently provide too much light. All plants must determine between what is the end of the day and what is simply a fluctuation of light due to environmental changes like animals, clouds, and other plants which are competing for light