I've had the same experience as others here. It's possible to have lucid dreams on less sleep, but it's much more rare. If I want them to happen without effort, which is how they've always happened for me, I need to sleep about nine hours first. It's like your mind gives your body's actual rest priority. Once you've had plenty of rest, THEN the fun dream stuff starts.

Or take naps. They happen all the time for me in naps.