Each night you cycle through the five stages of sleep. Even though you might assume nothing much is happening after you drift off, a lot is happening neurologically.

Stage 1
During the first stage, your sleeping lightly. Your eyes are closed and there’s a reduction in brain activity, but you can be awakened easily and if you are, you may not feel like you’ve slept at all. If you’ve ever had the sensation of falling and experienced a muscle contraction, that is stage 1 sleep. This period generally only lasts for five to ten minutes.

Stage 2
Once you enter the second stage, your eye movements stop and brain waves become slower. Peaks of activity, known as sleep spindles, begin to occur. Your body temperature will decrease and your heart rate will slow as you prepare to enter deep sleep.

Stages 3 & 4
These are the stages of deep sleep. Dreams are more common than in the first two stages; night terrors and sleepwalking can also occur. It’s difficult to wake people up during these deep stages of sleep; if you are awakened, you can expect to feel disoriented and groggy for several minutes before you adjust. There is no muscle activity or eye movement and the brain produces delta waves or slow waves. As you might imagine, stage 4 is a more intense version of deep sleep than stage 3.

REM Sleep
REM, or rapid eye movement, is a stage in which your eyes really move rapidly. This is the stage where you’ll have the most vivid dreams since your brain waves are similar to when you’re awake. Your hear rate and pace of breathing increases and muscles will be immobile as paralysis occurs. Your first round of REM sleep may only last 10 minutes, but the length of this stage increases throughout the night and may last up to an hour.