Whilst trying to sleep yes, however meditation during the day is best done with a straight back as it allows air to flow into your core.
I like to sit in a chair. Remove my shoes and feel the floor underneath my feet. Breathe and meditate. I think I may fall asleep if I lay on my back to meditate. But so what works for you
I do reflect mostly by sitting up and focusing my gaze on one thing to center myself when I reflect while lying on my back I know I am able to calm myself more and focus on my breathing
I personally prefer to meditate cross-legged, arms resting on my knees, and my eyes closed. I’ve never meditated a different way, therefore I cannot say whether or not meditating while lying on my back is valid or not.