It sounds like the brain will only sleep as deeply as it&#x27;s safe to. That lines up more with the results in the article than the notion that it&#x27;s directly about how many times you&#x27;ve been in that location before. When you&#x27;re in charge of a boat, you have to be alert for any danger. When you&#x27;re a fireman on duty, you have to be ready to hop into your pants. When you&#x27;re in a strange sleep lab and covered in biosensors, you&#x27;re going to be creeped out and on-edge.