Art Markman, Ph.D. Psychology Advisor replied almost 3 years ago:
If you get sleepy when meditating, then chances are you are sleep deprived. Sleep deprivation is one of the most important public health problems in the modern world. Sleep deprivation makes it harder for you to think, and it also makes you irritable and can increase anxiety. One reason that people meditate is to remove stress from their lives. If you are finding that you fall asleep when meditating, then perhaps some of that stress is being caused by sleep deprivation. Get to know how much sleep your body needs. Most people need at least 7 hours of sleep a night. Meditation is a practice. It is not something you can learn to do instantly. As you develop a practice of medication, you will learn more about your patterns of thinking. Don't assume that you will turn out the lights, put on a tape and suddenly be an expert in meditation. If you are worried about your own practice of meditation, though, there is probably a local yoga studio that offers some classes that teach meditation practice. Look for one near you.