Don't blame yourself for feeling sleepy during japa, as if you lacked determination — before real taste for the Name develops, drowsiness catches almost everyone, and it is usually not a matter of willpower but of the body. Gurudev puts it plainly: sit so your spine is a little raised off the ground — the spine should be straight. Otherwise prāṇa cannot flow, and you'll tend to get sleepy. Maya won't argue philosophy with you — she'll simply close your eyes the moment your spine goes slack. So sit cross-legged like a statue, without moving, slightly raised off the floor, so the breath stays free and the Name sounds in a clear, waking body.