Lying down is a traditional meditation posture, so yes you can. We generally recommend sitting because it offers a nice balance of focus and relaxation. Feel free to lie down if you feel like you’re able to sustain your attention without falling asleep. If it is comfortable for you, bend your knees so they point to the ceiling when you meditate lying down, it will help you be more alert. Additionally, you may bend an arm at the elbow so your fingers point to the ceiling. This also supports alertness. The benefit of this modification is that your hand on the floor is feedback that you fell asleep if it doesn't succeed in waking you up.