Roos Koole/Getty Images Some mornings when you wake up, your head is fuzzy, your body is heavy, and you don’t feel rested. It felt like you were dreaming all night. But did all that dreaming actually wear you out? Let’s look at what the science says. We all dream, but not everyone remembers it Most dreaming occurs during rapid eye movement (REM) sleep, which makes up 20–25% of our total sleep time. We have four to six rounds of REM throughout the night, with each round growing longer as mornin