There are four stages of sleep:

Stage 1. Non-rapid eye movement (NREM): You transition between being awake and being asleep.

Stage 2. NREM: Your body temperature drops and your heart rates slows.

Stage 3. NREM: This is your deepest sleep where your muscles relax and your brain activity slows down.

Stage 4. Rapid eye movement (REM): Dreams occur and your body becomes immobilized, but your brain activity increases. Your eyes move rapidly.

Sleep paralysis occurs during stage four. During REM, your body is paralyzed so you do not act out your dreams. During sleep paralysis, your body remains paralyzed, but your mind wakes up. Because you are technically in the dream stage of sleep, you may also hallucinate sensations such as feeling pressure on your chest or limbs, seeing a shadowy figure, or even have an out-of-body experience. Sleep paralysis can be scary because you are unable to move. In some cases, it feels like you are unable to breathe.

Nightmares, like sleep paralysis, occur during REM. They are disturbing dreams that you can often remember in detail. If they become frequent, they can lead to something called sleep phobia. This occurs when you do not want to go to sleep because you are afraid of having a nightmare.

Night terrors occur in deep sleep or NREM stage three. During night terrors, the front part of your brain that controls executive functioning and memory is asleep while the back part that controls motor movement is awake. This is similar to sleepwalking. During a night terror, the sympathetic nervous system, which controls your "fight-or-flight" response, is unusually active. A person often does not remember that they had a night terror, but they will go into fight-or-flight mode in their sleep, experiencing an increased heart rate and blood pressure. They may also have a terrified look on their face. Night terrors are common among young children, who tend to outgrow them by middle school.

Sleep paralysis, nightmares and night terrors do not affect sleep quality. Often, simple reassurance that everything is OK is enough to help someone during one of these sleep disturbances. These issues can also get worse if you are sleep deprived. This is why having good sleep habits, including getting enough sleep and not drinking alcohol close to bed time, is so important.

• Sleep Medicine Physician Hrayr P. Attarian, MD