I thought that if I could use either a little or little in these examples; She wanted little sugar in her tea. She wanted a little sugar in her tea, then in this question too I only want /a/_ little sugar in my tea, please,I could use either.
The sentence has to be: "I only want a little sugar in my tea, please." The sentence, "I want little sugar in my tea.", is possible if you're very angry and want to sound belligerent.
"Nobody knows why people talk or walk in their sleep." is correct.