During non-REM sleep (about 80% of an adult’s sleeping time), you breathe slowly and regularly. But during REM sleep, your breathing rate goes up again. That’s the time we typically dream. Breathing also becomes more shallow and less regular during this sleep phase. Some of it may be due to throat muscles relaxing or to less movement of your rib cage during REM sleep. Whenever you’re sleeping, your oxygen levels are lower and your carbon dioxide levels are higher because your level of breathing goes slightly down.