If you fall asleep sitting down, you could always try standing up while you meditate. Or just make yourself progressively umcomfortable enough sitting down to stay awake. However, if you fall asleep standing up then....please, see a doctor, because you probably have narcolepsy
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On the other side of the coin, if you fall asleep so easily, you have a unique oppurtunity to get into an ultra deep state of conscious meditation known as Lucid Dreaming. Simply set an alarm clock for 15 minute intervals and let yourself drift off focusing on the idea that: no matter what I see from this point on till the time the alarm goes off is a dream. Keep practicing that and lucidity will be yours in no time. After you get in the habit of being lucid whilst your body is unconscious, I doubt falling asleep will be your opponent any longer. Cheers.