There are several reasons people can feel sleepy when mediating. The fundamental key is that you need to learn to be in control to stay master of your mind and body, instead of your mind controlling your body.
If you have had an overwhelming day or struggle with stress, then you may discover that you frequently become sleepy during the course of your meditation, as your body starts to destress from the pressures of your life. If you routinely experience this, then take some time-out to stand or splash some water on your face prior to proceeding.
During the afternoon, many people experience an energy slump, so instead, they choose to practice meditation in the morning when the body is awake. It is also common that a relaxed sitting posture can contribute to the impulse to nod off, so take the opportunity to get started with some good posture before you begin your meditation.
A far more perplexing reason why you might become sleepy during your meditation is because your mind controls your life. You may uncover that it is your mind which is wielding control, drawing you away from your meditation. Essentially your mind exerts control over you, impeding your ability to effectively practice meditation. As you continue to meditate, you can strive to take back control from your mind, nevertheless be aware that your mind is going to resist relinquishing control at any cost.