The stages of sleep
Stage 1 is the transitional stage between wakefulness and sleep. In Stage 1 sleep, you shift in and out of consciousness, and lose a sense of time and place. Sleep onset – the process of falling asleep – takes place during Stage 1. It is easy to be awakened from this stage of sleep. If awakened from Stage 1 sleep, you might not even be aware you’d been sleeping.
Stage 2 is a phase of light sleep. Over the course of a night and several sleep cycles, you will spend approximately 50 percent of your time in Stage 2 sleep as you move in and out of REM and the deepest stage of NREM sleep. During Stage 2 sleep, brain activity slow from waking levels. The body also relaxes physically, as heart rate and breathing decrease. Stage 2 is a state of full sleep, but not deep sleep: it is still easy to wake from this stage. During both Stages 1 and 2, the body relaxes as is prepares to move into deeper phases of sleep.
Stage 3 is a phase of deep sleep, also known as slow-wave sleep. Typically it takes about 30 minutes to reach Stage 3 sleep for the first time after falling asleep. During this phase of sleep, brain waves slow considerably. Heart rate and breathing slow, blood pressure lowers, and muscles relax. Stage 3 is a critical time for physical restoration. Repair occurs at the cellular level, restoring strength and function to tissue, muscle, and organs throughout the body. During Stage 3 sleep the body also turns its attention to restoring function to the immune system.
You reach REM sleep for the first time approximately 90 minutes into the night. Periods of REM sleep start as brief, but REM sleep gets progressively longer throughout the night. During REM sleep, the brain increases its activity levels significantly compared the other sleep phases. Most dreaming occurs during REM sleep. If you wake with an awareness of having been dreaming, you likely awoke from REM sleep. REM sleep is a critical phase of sleep for learning and memory, a time when the brain processes and stores information.