What You Need to Know About Sleep Cycle

What is Sleep

First of all, along with food and water, sleep is part of our daily routine that is just as important to survival. And we spend nearly a third of our lives sleeping. Most of all, the brain plays a significant role in sleeping. In fact, the brain uses, what is known as a sleep cycle, to:

  • First of all, store your day’s experiences
  • Also, form or maintain neural pathways that lets you learn and create new memories
  • Finally, release hormones that regulate your energy, mood, and mental acuity

Most noteworthy, your brain and body are very active while you sleep. Above all, sleep removes toxins that build up in your brain while you are awake. Also, sleep affects your entire body including your brain, heart, and lungs. Finally, sleep affects your metabolism, immune functions, mood, and disease resistance.

Components of a Sleep Cycle

First and foremost, there are two types of sleep: Rapid Eye Movement or REM sleep and Non-REM (NREM) sleep. Furthermore, non-REM sleep is made up of three stages: Stage 1, Stage 2, and Stage 3. Moreover, each stage and type of sleep is linked to specific brain waves and neuronal activity. So, a complete sleep cycle is composed NREM Stage 1 sleep, NREM Stage 2 sleep, NREM Stage 3 sleep, and REM sleep.

NREM Stage 1

First of all, as the first component of a sleep cycle, NREM Stage 1 sleep is a transition from being awake to being asleep. And, this stage is usually very short, lasting maybe seven minutes. Most of all, in this light sleep, we drift in and out of sleep and can be awakened easily.  And, awakened people often remember fragmented visual images. While, others experience sudden muscle contractions often preceded by sensations of falling. Above all in this stage the following occurs:

  • First, the heart rate decreases
  • Also, the breathing rate slows down
  • In addition, eye movements slow down
  • And, your muscles relax
  • Also, within minutes of nodding off, your brain produces alpha and theta waves
  • Finally, the brain waves slow down

NREM Stage 2

Next, the following occurs in the second component of a sleep cycle, NREM Stage 2 sleep:

  • First, the heart rate decreases further
  • Also, the breathing rate slows down even further
  • In addition, eye movements stop
  • And, your muscles get more relaxed
  • Also, the body temperature drops
  • Finally, the brain waves slow down. In addition, the brain waves are interspersed with sudden, intermittent, increases in brain wave frequencies called sleep spindles.

NREM Stage 3

Stage 3 or 4

Actually, a sleep cycle used to have a Stage 3 and a Stage 4. But because of their similarity they were merged into just one stage. From here on, this one stage of the sleep cycle is referred to as NREM Stage 3. And NREM Stage 3 is where deep sleep occurs.

NREM Stage 3 Activity

Meanwhile, the following takes place in the third component of a sleep cycle, known as NREM Stage 3 sleep:

  • First, the heart rate slows to its lowest level - nearly 30% below your awake value.
  • Also, the breathing rate slows to its lowest level
  • In addition, there is no eye movement
  • And, your muscles are totally relaxed
  • Also, the blood supply to the muscles increases
  • Next, your blood pressure drops - nearly 30% below your awake value
  • In addition, it is very difficult to wake up someone in this stage. In fact, people awakened, in this stage, do not adjust immediately to becoming awake. Furthermore, they often feel groggy and disoriented for several minutes after being awakened.
  • Most of all, this is a restorative stage when the body repairs muscles and tissues, stimulates growth and development, boosts immune functions, and builds up energy for the following day.
  • First of all, new tissue growth occurs at the cellular level
  • Also, the pituitary gland releases Human Growth Hormone (HGH). Moreover, HGH is released in pulses during deep sleep. And, in adults this helps the body maintain and repair itself. Also, in adults, HGH promotes cell repair that is necessary after the stress of weight training.
  • In addition, production of proteins increases.
  • Especially relevant, recent neural network research indicates that deep sleep may be important in helping to clear the brain for new learning the following day.
  • Finally deep sleep is extremely effective at decreasing the sleep drive that builds up steadily over the day. By rapidly reducing this sleep need, deep sleep makes you feel most refreshed in the morning.


Above all, it is in this stage that sleepwalking happens. As a matter of fact, this happens when there is a sudden arousal from deep sleep that causes the motor centers of the brain to awaken. As a result, the person is in a sleep state characterized by complex motor activity with limited judgment and awareness.

NREM Stage 3 Sleep Periods

Specifically, most NREM Stage 3 deep sleep occurs in the first two to three hours of sleep – or, in effect, the first two sleep cycles. Furthermore, most of it typically occurs in the first cycle. And, as the night progresses, the amount of deep sleep decreases. In fact, as the night progresses, deep sleep is replaced by the lighter NREM Stage 2 sleep and increasing amount of REM sleep.

Daytime Nap

Because there is not enough time to cycle into deep sleep, a 20 to 30 minute afternoon nap usually does not affect nighttime sleep. In fact, most 20 to 30 minute afternoon naps are usually in NREM Stage 2. However, afternoon naps longer than 30 minutes may end up taking you into NREM Stage 3 sleep. And because NREM Stage 3 sleep decreases the sleep drive, you may have a hard time falling asleep at night.

Factors Affecting Deep Sleep

Most of all, the ability to get sufficient amount of deep sleep is vulnerable to the effects of stress, sleep disruption, aging, and many drugs.

In adults, women usually get longer slow wave sleep than men. And, women ages 70 and older get an adequate amount of stage 3 sleep (15% of the total sleep time). On the other hand, men of the same age often get only one cycle of deep sleep (about 5% of the total sleep time).

Because the circadian rhythm changes as we get older, in general, the amount of NREM Stage 3 sleep falls.

Consequences of Insufficient Deep Sleep

By preventing sufficient deep sleep, the above factors contribute to the run-down feelings of many people. Most of all, these are folks who daily face financial pressure, have sleep apnea, are growing old, or taking certain drugs.

Most of all, when we don’t get enough deep sleep we lose out on healing benefits of deep sleep. And these benefits include the ability to repair damage and build replacement tissue (such as skin and bone cells). Furthermore, without these opportunities to keep strong skin, bones, and organs we age faster. In addition, we are more prone to heart disease, osteoporosis or neurological conditions.

Most noteworthy, slow-wave sleep is the most restorative to your body. As a result, interruptions in slow-wave sleep are the most damaging to your metabolic health. And these interruptions along with other consequences of sleep deprivation occur when you don’t sleep 8 hours.

Help for Getting Sufficient Deep Sleep

So to get sufficient amount of deep sleep, ensure you have a regular pre-midnight bedtime, get any apnea problems treated and use relaxation breathing or meditative techniques to slip into sleep.


Above all, as the fourth component of a sleep cycle, REM sleep occurs about 90 minutes after falling asleep. Also, each REM stage can last up to 60 minutes. And the following occurs in REM sleep:

  • First of all, your heart rate increases to that when you are awake
  • Also your breathing becomes faster, irregular, and shallow
  • In addition, your eyes, hidden by your closed eyelids, move rapidly from side to side as well as up and down
  • Next, your blood pressure also increases to that when you are awake
  • Also, your arm and leg muscles are temporarily paralyzed – to prevent you from acting out your dreams
  • Next, REM sleep is associated with increased production of proteins
  • In addition, males may get penile erections
  • Also, mixed frequency brain wave activity becomes closer to that when you are awake
  • And you dream. Incidentally, people who wake up in this stage often describe bizarre and illogical stories. Especially relevant, we typically spend more than 2 hours each night dreaming.
  • Since REM sleep is when your brain consolidates and processes information from the previous day, it plays an important role in learning and storing information in your long term memory.
  • Finally, REM sleep also nurtures cognition and problems solving. In fact, studies have shown that people learning a new physical task will improve their performance overnight, but only if they get sufficient REM sleep.

Factors Affecting REM Sleep

First of all, as you get older, the duration of your REM sleep becomes shorter. Since, memory consolidation occurs in both REM and NREM sleep, shorter REM sleep means memory consolidation suffers.

In addition, while alcohol helps people fall into light sleep, they lose REM and the deeper, more restorative stages of sleep. Instead alcohol keeps people in lighter stages of sleep from which they are easily awakened.

 Also, while in REM sleep, some people lose ability to regulate body temperature. As a result, abnormally hot or cold temperatures, in the surroundings, disrupts their REM sleep. Consequently the benefits of REM sleep suffer.

Sleep Cycle Dynamics

Each of the sleep cycle components can last anywhere from 5 to 15 minutes. Also one complete sleep cycle lasts anywhere from 90 to 110 minutes. Therefore, in a normal 8 hour sleep, the brain completes about 5 sleep cycles in 90 x 5 = 450 minutes or 7.5 hours.

However, the first half of the night is essentially NREM sleep where the REM portion is short. And the second half of the night is essentially REM sleep where the NREM stages are short. In addition, towards the end of your sleep, you are almost completely in REM sleep with bits of stages 1 and 2. Unfortunately it’s not so simple. In fact, you get more NREM sleep between 11 PM to 3 AM and more REM sleep between 3 AM and 7 AM. So if you are an early sleeper you will get more NREM sleep and less REM sleep. And conversely, if you are a late sleeper you get less NREM sleep and more REM sleep.

Also as people transition from childhood to adults there is a reduction in deep sleep (NREM Stage 3). In fact, 40% of NREM Stage 3 sleep duration gets replaced by NREM Stage 2 sleep in adults. In addition, as adults get older they get less REM sleep.

Most noteworthy, 50% of the sleep is in NREM Stage 2, 25% in REM, and 15% to 20% in deep sleep. Moreover, most of the deep sleep occurs in the first third of the night.

By the time we're 60 year old, we're only sleeping 6.2 hours a night. And REM sleep has fallen to about 75 minutes; Deep sleep to less than 40 minutes; and on a typical night, we're awake for an average of 44 minutes.

Sleep Cycle Realities

Paradoxically, people wake up regularly during the night because people sleep in cycles. In fact, these cycles become shorter as we get older. Also, as we get older, we aren't able to sustain deep sleep as much as we used to. But mostly when we wake up after a cycle, we will roll over or shuffle or go to the bathroom and then try to go back to sleep again. However, for some people, it's not the waking up that's the problem, it's getting back to sleep.

How the Brain Does All This

First and foremost, the brain’s hypothalamus controls your sleep. Also, the brain’s stem communicates with the hypothalamus to control the transition between being asleep and being awake.

In addition, REM sleep begins with signals from an area of the brain’s stem called the pons. And, these signals travel to the thalamus which relays them to the cerebral cortex. Most of all, the cerebral cortex is the outer layer of the brain that is responsible for learning, thinking and organizing information.

In addition, the pons send signals that shut off neurons in the spinal cord, causing temporary paralysis of the limb muscles. As a result, we don’t act out our dreams.

Above all, the thalamus acts as a relay of information from the senses to the cerebral cortex. And the thalamus allows you to tune out the external world. Most noteworthy, during REM sleep, the thalamus is very active, sending the cortex images, sounds and other sensations that are part of the dream.

In addition, the pineal gland (located within the brain’s two hemispheres) increases production of the hormone melatonin, which helps you sleep once the lights go out.