Ema S N.
I would say both, as there are different types of meditation. If you start and end your day with meditation it will keep you calm and at peace throughout the day, but maybe if you don't have time in the morning, you should definitely do it at night. Personally i prefer meditating right before I go to sleep, to shut down my brain and to stop all the stress and tension but it depends on you. If not meditation, starting your day with breathing exercises helps me a lot too, maybe it'd work for you maybe not, give it a try though.