Why my Mind told me to Sleep?

I have observed one thing, when i meditated first time Vipassana last week, i saw vibrations in my body, but after that day its been many days i have not seen them, in-fact i can not sit for long (only 15-20 minutes), its not that my legs hurt, But its my mind, it says “Why are you doing this, lets sleep, when i tell it no, it says its more comfortable to meditate in sleep”. and then i sleep, How do i control it, am i missing something ?