When we talk about sleep, I think there are really two big questions that matter: Did you sleep long enough? And: Did you sleep well? We can view these as the two dimensions of sleep: Duration and Quality. Most people understand the first one. If you're routinely sleeping five hours a night, there is a fairly obvious problem. The quality of sleep is more complicated as it depends on many things. Are you repeatedly waking up? Are you breathing normally? What is your sleep env