The Stages of Sleep

In order to understand about what you can do to help improve your sleep, you need to understand sleep itself. In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. During sleep, the body cycles between REM and non-REM sleep.

It’s in the REM stage of sleep that dreams generally occur. The first REM sleep period usually occurs about 70 to 90 minutes after you fall asleep. REM sleep begins with signals from an area at the base of the brain called the pons. These signals travel to a brain region called the thalamus, which relays them to the cerebral cortex – the outer layer of the brain that is responsible for learning, thinking, and organizing information. The pons also sends signals that shut off neurons in the spinal cord, causing temporary paralysis of the limb muscles. Breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake, heart rate increases, blood pressure rises, and the body loses some of the ability to regulate its temperature. REM sleep accounts for 20–25% of total sleep time in normal human adults, but infants spend much more time in REM sleep. Some experts believe that this greater time is due to the fact that REM sleep stimulates the brain regions used in learning, which may be important for normal brain development during infancy. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening. The final REM period may last up to an hour.

According to the American Academy of Sleep Medicine standards, non-REM sleep consists of three stages: N1, N2, and N3 (prior to 2007, non-REM sleep was considered to have fours stage – Stage 1, Stage2, Stage 3, and Stage 4).

Stage N1 is the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. Sometimes referred to as somnolence or drowsy sleep, this stage lasts about five to 10 minutes. During this stage you may experience sudden muscle contractions called hypnic myoclonia, often preceded by a sensation of starting to fall. Some people may also experience hypnagogic hallucinations during this stage, and you lose some muscle tone and most conscious awareness of your external environment.

Stage N2 is a period of light sleep. During this stage, polysomnographic readings show bursts of brain activity ranging from 12 to 16 Hz and K-complexes. Mmuscular activity as measured by EMG decreases during this stage, you lose complete conscious awareness of your external environment, your heart rate slows and your body temperature decreases. At this point, the body prepares to enter deep sleep. This stage occupies roughly 45 to 55% of total sleep in adults.

Stage N3 is also known as slow-wave, delta, or deep sleep. This stage is characterized by delta waves ranging from 0.5 to 4 Hz (also called delta rhythms). This is the stage where parasomnias such as night terrors, bedwetting, sleepwalking, and sleep-talking occur, particularly in children. If you’re awakened during this stage, you won’t be able to immediately adjust to being awake, and will often feel groggy and disoriented for several minutes. During this stage the body repairs and regenerates tissues, builds bone and muscle, and appears to strengthen the immune system.

A complete sleep cycle takes 90 to 110 minutes on average. The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stages N1, N2, and REM.

If you’re awakened after sleeping more than a few minutes, you’re usually unable to recall the last few minutes before you fell asleep. This sleep-related form of amnesia is the reason people often forget telephone calls or conversations they've had in the middle of the night. It also explains why you often don’t remember your alarm ringing in the morning if you go right back to sleep after turning them off.
Hopefully you now have a better understanding of the different stages of sleep. By understanding how sleep is supposed to work, you can better understand the different problems that can interfere with proper sleep, which is the first step to help you correct these problems.