There are two distinct types of sleep: rapid- eye-movement (REM) sleep (when most dreaming occurs), and non-REM sleep.
Typically, when you fall asleep you begin in non-REM sleep. Most people spend about 80% of the night in this type of sleep.
During non-REM sleep your heart rate, breathing, and blood pressure all drop to levels below those that occur while you are awake. During REM sleep – approximately 20% of your time asleep – both your blood pressure and heart rate can go up and down. Any time you wake up from sleep (even briefly), your heart rate and blood pressure climb, and your heart must work harder. When you wake up in the morning, your blood pressure and heart rate both go up and then stay at a higher level throughout the day.