The Basic Sleep Stages: Non-REM and REM Sleep Cycles

Understanding sleep stages can be very important for people whose sleep cycles feel incomplete or interrupted. Anyone who has ever slept for hours on end, but still awakened feeling tired knows that not all sleep is the same.

sleep cycles and sleep stages

Sleep is divided into two major types :

  1. NREM – The first is non-rapid eye movement sleep, or non-REM. This is divided into four sleep stages, each one a little deeper than the previous one.
  2. REM – After the non-REM stages are completed, people move into the second type of sleep, rapid eye movement, or REM, sleep.

During this REM sleep, most dreaming occurs and the eyes move quickly back and forth. People who don’t experience all these stages during a given sleep cycle can awaken tired and disoriented.

Early Sleep

The earliest phases of sleep can be hard to detect, since the sleeper still feels relatively awake. The brain, however, has already begun to stop producing as many tiny, fast beta waves and has increased its production of the slower alpha waves.

During this period, most people don’t quite feel asleep and may have thoughts that feel dreamlike.

Some sleepers even experience very vivid sensations and images while in this state. These are called hypnagogic hallucinations and may suddenly cause you to wake up. Some people hear voices, feel as though they are falling, or are struck by the sudden sensation that a person or animal is in the room with them.

  • Stage One SleepSleep cycles begin with stage one sleep, which is fairly light and acts as a transitional phase between being awake and being asleep. The brain starts producing very slow waves during this sleep stage. This type of brain wave is known as a theta wave.
  • Stage Two Sleep – This part of most people’s sleep cycles is much longer than stage one and lasts for about 20 minutes. The brain’s wave pattern begins to become rhythmic and repetitive, producing a signature known as a sleep spindle. This encourages the heart rate to slow down and the body temperature to drop.
  • Stage Three Sleep – Delta waves, extremely slow and deep brain waves, are produced during stage three. This phase is usually fairly short and acts as the transition between lighter sleep and deep sleep stages.
  • Stage Four Sleep – Often referred to as delta sleep, stage four is characterized by large numbers of delta waves. This stage lasts for about half an hour in most people. It is the phase in which people who suffer from bed-wetting or sleepwalking are most likely to have problems.
  • Stage Five Sleep – By stage five, the body is ready to enter the REM phase, during which breathing and brain activity speed up. Muscles relax more and sleep paralysis sets in. Most people enter this stage about an hour and a half after beginning sleep, with progressive REM sleep stages lasting longer as the sleep cycles continue.

It’s important to pass through all these phases for healthy sleep. Non-REM sleep stages allow people to build and repair tissues and develop a healthier immune system, while REM stages are important for mental health.

When people get older, their sleep becomes lighter and less deep. Aging is also linked to shorter durations of sleep, although, contrary to public opinion, studies have shown that the necessary amount of sleep remains the same with the increase of age.

Presumably, this would account for the increasing frequency of napping throughout the day with seniors. The body is just trying to compensate for the diminished quality of sleep during the night.



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