I'm watching them sleep
What is so interesting about them while they're sleeping?
I love observing people and testing them.
Unless you have a medical reason to do so, it's creepy to test things on people and watch them while they sleep without their permission. If you are curious about someone you should talk to them directly to understand what is going on rather than try to watch them while they sleep and guess.
I'm always careful about the things I do, don't judge me.
Unless they have agreed to this you know they might not feel the same way. Can you imagine how it might be if someone were spying on you as you slept?