When we sleep, we cycle between two major categories of sleep – REM (rapid eye movement) and non-REM - on an average of every 90 minutes. During non-REM or "quiet" sleep, a person progresses through stages of increasingly deep sleep. Body temperature drops, muscles relax, and heart rate and breathing slow.

The deepest stage of quiet sleep produces physiological changes that help boost immune system functioning. This deep sleep is when your body rebuilds and repairs. As we age, we get less non-REM sleep; on average adults experience just 30 minutes of this restorative sleep nightly. 

REM sleep, on the other hand, is the period when people dream. Body temperature, blood pressure, heart rate, and breathing increase to levels measured when people are awake but the body's muscles are relaxed to the point of paralysis. Interesting fact: unlike humans, dogs are not completely paralyzed during REM sleep and are known to run in place while dreaming.

REM sleep enhances learning and memory, and contributes to emotional health in complex ways. Research is discovering that sleep disruption affects levels of neurotransmitters and stress hormones, which wreaks havoc in the brain, impairing thinking and emotional regulation.