Usually, Upanishad is not chanted after sunset. It is said that Tiruvaimozhi is the essence of Upanishad. Why then there is no such restriction for Tiruvaimozhi recitation?

Upanishad is not chanted during the night times. As per the practice of elders, Tiruvaimozhi was recited during the night times. As an example – curd must not be partaken in the night time; but we partake ghee, which is an essence of the curd. Similarly with the recitation of Upanishad`s essence Tiruvaimozhi.