I use both, though no, you don't NEED both. I have a heat lightbulb for night but that is because the room temp fluctuates at night and it gives it an extra boost if it gets cold. I usualy either turn the mat off of way down for brumation and leve the light on for a little heat. Do you have a thermostat on your mat? If you don't, they can get up to 90 F.
You can use both, though I recomend turning the light off in the day if it is warm in the day where you live.