Ever since I was a kid I loved the idea of sleep learning: that you could put on headphones when you went to sleep and have conversational French when you woke up. I even tried it in college when, depressed by my mounting student loan debt, I decided I was going to go for a 4.0. All I got for my trouble was a few restless nights and a headache. Today, science can’t promise you passive language learning, but there may be some hope for the lazy learners among us. A new study published in Nature demonstrates a fascinating phenomenon: sleep conditioning, a la Pavlov. Here’s how it works.