This entirely depends on your preferences and what you want to see on your Karnataka trek. If you want to enjoy the warmth of the sun and see the lush landscapes stretching towards the horizons, then you can go for a day trek.
And if you want to experience the thrill of trekking in the dark and immerse yourself in the nocturnal sounds of the nighttime forests along with the splendid view of the starry sky and shimmering villages in the distance, then you should head for a night trek in Karnataka.