It can happen, especially if you are tired or meditate while lying down. Sleep and meditation are not the same, however. When you fall asleep, the conscious attention that is an essential part of meditation eventually disappears too.
If you frequently fall asleep when you actually want to meditate, try a more upright posture, open your eyes a little, or meditate at a time of day when you are more awake. And if you are simply very tired? Your body may need sleep rather than meditation right now.