Sleep is marked by dynamic changes throughout the body. It’s made up of different phases, and as you move through them, your breathing, blood pressure and body temperature will all fall and rise. Tension in your muscles mostly stays the same as when you are awake – except during REM phases, which account for up to a quarter of your sleep. During these, most major muscle groups ease significantly. But if your throat muscles relax too much, your airway collapses and is blocked. The result is obstructive sleep apnoea – from the Greek ápnoia, or "breathless".