Both. You should start your practice ideally with at least a few minutes of meditation to get situated for your practice. This can be part of focusing on breath and setting intention to get started. Mainly bringing awareness to breath, introducing a breathing pattern such as 4-4-4-4 breathing.(in 4, hold 4, out 4, hold 4, repeat) It doesn’t have to be long, but should calm you down like you suggest. Then afterwards do a longer meditation. It shouldn’t make you sleepy. Remain aware during it and feel the results of your practice. It should keep you alert. Just keep coming back to the breath if you get or another point of focus of your choosing, such as the point between your eyes, or i personally like to meditate on “feelings” and try to feel a feeling as much as i can, like meditating on bliss. Yes it is for relaxing but relaxing the mind not the body necessarily. Meditation is an active practice that requires focus, it’s not necessarily “rest”. Also, you don’t have to do it right after practice. That’s also what savasana is for. You can meditate any time of the day