By feeling sleepy when meditating, your body is telling you that it is relaxed. This makes it a very common challenge for both beginner and seasoned meditators. In the Staying on Track meditation series, you can find a meditation titled Sleepiness that we recommend for a meditation technique that can help with this.
Tamara, our Head of Mindfulness, also wrote an article on our blog that includes 6 tips for fighting off your body's tendency towards sleep while trying to meditate. You can check it out here: http://blog.calm.com/new-blog/2016/8/19/qa-with-tamara-how-to-combat-sleepiness-during-meditation