Sleep usually comes in cycles consisting of five stages: stages 1 and 2, stages 3 and 4, and the REM (rapid eye movement) stage. During sleep, a person goes through about five to six cycles.

Stages 1 through 4 are non-REM stages. A combination of REM and non-REM sleep partly determines the quality of sleep you get.

Non-REM sleep ranges from very shallow (stages 1 and 2) to very deep (stages 3 to 4). Stages 3 and 4 envelop you in baby-like sleep; when you wake up during this stage, you feel very rested and relaxed.

With REM, the dreaming stage, your brain is active while your entire body is “paralyzed.” That is a protective mechanism: you don’t want to be dreaming about jumping off a building roof top and your body exactly doing that.

Markedly in adults, sleep quantity may stay the same, but sleep quality changes. As we age, stages 3 and 4 decline and the time we spend awake in bed gets longer. Other factors that may affect sleep quality are snoring and sleep apnea, medical conditions that need the attention of a doctor.