What happens during SLEEP

A lot happens in your body while you sleep. When you get your ZZZs, you cycle between REM and non-REM sleep.

REM stands for rapid eye movement. During REM sleep, your eyes move quickly in different directions. That doesn't happen during non-REM sleep.

What is REM sleep?
The brain cycles through five distinct phases during sleep: stages 1, 2, 3, 4, and rapid eye movement (REM) sleep. REM sleep makes up about 25% of your sleep cycle and first occurs about 70 to 90 minutes after you fall asleep. Because your sleep cycle repeats, you enter REM sleep several times during the night.

During REM sleep, your brain and body are energized and dreaming occurs.

REM is thought to be involved in the process of storing memories, learning, and balancing your mood, although the exact mechanisms are not well understood.

REM sleep begins in response to signals sent to and from different regions of the brain. Signals are sent to the brain's cerebral cortex, which is responsible for learning, thinking, and organizing information. Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles ("paralysis") in the arms and legs. Abnormal disruption of this temporary paralysis can cause people to move while they are dreaming. For example, this type of movement while dreaming can lead to injuries that could happen when a person runs into furniture while dreaming of catching a ball.

REM sleep stimulates regions of the brain that are used for learning. Studies have shown that when people are deprived of REM sleep, they are not able to remember what they were taught before going to sleep. Lack of REM sleep has also been linked to certain health conditions, including migraines.

Men have several night-time erections
On average, a healthy man has 3 to 5 erections during a full night's sleep, with each erection lasting 25-35 minutes. It's common for men to wake up with an erection, informally called a "morning glory". This is in fact the last in the series of night-time erections. The cause of night-time erections isn't fully understood. However, studies suggest they are closely associated with the phase of sleep known as REM (rapid eye movement) sleep. This is when dreaming is most common. Whatever their cause, most doctors agree that night-time erections are a sign that everything is in working order.

The reason for dreaming during REM sleep is not understood. While some of the signals sent to the cortex during sleep are important for learning and memory, some signals seem to be random. It is these random signals that may form the basis for a "story" that the brain's cortex tries to interpret or find meaning in, resulting in dreaming.

What Happens During Non-REM Sleep?
There are three phases of non-REM sleep. Each stage can last from 5 to 15 minutes. You go through all three phases before reaching REM sleep.

Stage 1: Your eyes are closed, but it's easy to wake you up. This phase may last for 5 to 10 minutes.

Stage 2: You are in light sleep. Your heart rate slows and your body temperature drops. Your body is getting ready for deep sleep.

Stages 3: This is the deep sleep stage. It's harder to rouse you during this stage, and if someone woke you up, you would feel disoriented for a few minutes.

During the deep stages of NREM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system.

As you get older, you sleep more lightly and get less deep sleep. Aging is also linked to shorter time spans of sleep, although studies show you still need as much sleep as when you were younger.