I had that problem at first, too. Meditating at the end of the day is bad for that, and you must not lie down if you have this problem. Sit with your back straight, and remember that your mind is supposed to be alert, not relaxed. Since I started the straight-back and alert mind approach, I have never fallen asleep meditating. It's calm, alertness, noticing everything, but not reacting to it. It's a good state for learning, so it makes sense to cultivate it in meditation. It's a good state in which to live. It's not about relaxing.

If you need to sleep, sleep, and then meditate afterwards. Early in the day is a good time.