I keep falling asleep during yoga nidra — is that ok?

It depends on the mode. If you’re doing sleep preparation nidra: great, falling asleep is the goal. If you’re doing daytime restoration: falling asleep is slightly less optimal because you miss the theta-state awareness phase where the deepest restoration occurs, but it’s still restorative. Tips for staying in the sweet spot: keep the room slightly cooler, use a slight incline instead of lying flat, or try seated nidra. With practice, you’ll learn to hover in that edge state.