What is a Sleep Cycle ?

Until just a few decades ago, scientists understood very little of what was happening to people when they fell asleep. It was assumed that the brain simply shut off and not much was going on. In reality, however, your brain is going through a whole host of rest, restoration, and cleansing processes during sleep that help it to function better the following day.

Understanding the way that your brain works within the cycles it goes through during sleep can provide you with helpful insight. If you can understand what your body and brain are accomplishing during the night, then you can maintain better sleeping habits and promote a healthier lifestyle altogether.

How Sleep Cycles Work

Sleep cycles are a series of stages that your body and brain are taken through beginning with light sleep, heading into deeper sleep, and eventually ending up with REM sleep. These stages usually last between 5 and 15 minutes each, and the completion of the journey through the stages lasts between 90 and 120 minutes and is considered to be a full cycle. In a full night of 7-8 hours of sleep, you will usually go through 4-5 complete sleep cycles.

5 Stages of the Sleep Cycle

Although scientists used to consider that there were 5 stages of the sleep cycle, in recent years they have grouped stages 3 and 4 together, leaving us with just four remaining sleep stages for a complete cycle.

The stages of the sleep cycle are as follows:

Sleep Stage One

When you’re nodding off to sleep, Stage 1 occurs, which is the lightest stage of sleep. During this time it is easy for a person to be woken up and it often lasts for about seven minutes. Many people may experience physical twitches or startle reactions during this phase as the brain’s motor areas are being stimulated at this time. Alpha and theta waves are being produced during this stage and eye movement slows.

Sleep Stage Two

At this point, your sleep is deeper than Stage 1 but still fairly light. This is a stage that you’ll be in if you take a short, 20-minute-or-so power nap. During this stage the brain wave frequency increases and then slows down, producing what is known as sleep spindles.

Sleep Stage Three (and Four)

Sleep begins to deepen during this very deep stage of sleep as the slower delta waves are produced by the brain. There is typically no muscle activity or eye movement during this stage. The body and brain are less responsive to any outside stimulus during this time and waking someone up at this time may feel almost impossible. If you do wake up during this time, you might feel a disoriented and/or unable to move for a few minutes.

During this stage of sleep the body and brain are doing a great deal of repair and restoration work. Human growth hormone is released during this stage which repairs tissues and muscles and stimulates development. This is the stage in which the immune function is boosted and the energy for the next day is beginning to build.

Parasomnias also tend to happen during this deep sleep stage, including activities such as bedwetting, talking during sleep, sleepwalking, or experiencing night terrors.

If a person does not go through enough of this deepest stage of sleep each night, they may wake feeling exhausted and as if they haven’t slept at all. This is especially true for people who have sleeping disorder that wake them often throughout the night, or if they take medication that helps them enter into stages 1 and 2 of sleep but does not actually allow them to sleep deeply.

REM Sleep Stage

Most people know about REM sleep as the stage in which we dream. This final stage of the sleep cycle includes bursts of rapid, jerking eye movement when they dart up and down, back and forth. Although it’s possible that these eye movement are somehow related to the dreams that the person is having, there is no conclusive evidence for or against this.

During REM sleep, the body slows significantly and the movement muscles are basically paralyzed. The breathing rate, heart rate, and blood pressure all tend to increase during this time, while the body is less able to keep its temperature regulated during this phase.

One sleep cycle typically flows something like this:

Stage 1—Stage 2—Stage 3—Stage 2—Stage 1—REM Sleep

Going through all of these stages will usually take about 90 minutes for the first cycle and between 100 and 120 minutes through the rest of the night. Since the stages normally end with REM sleep, if a person is awoken at this stage, they may remember their dreams vividly.

Sleep Cycles and Time of Night

As scientists continue to study sleep cycles, they’ve learned that the whole business of sleeping is much more complicated than originally believed. As mentioned previously, the way that sleep cycles are experienced is affected by whether it is the first or subsequent sleep cycle of the night. This can also be impacted by the time of night that it is when sleep is happening.

It has been discovered that people spend most of their time in deep, non-REM sleep (Stage 3) for the first two or three cycles of the night. After that, for the final two or three cycles, people spend more of their time in REM sleep and lighter non-REM sleep (stages 1 and 2). That means that the deep, restorative sleep is happening first.

In addition, no matter when they fall asleep, most people get more non-REM sleep in the earlier half of the night (between 11pm and 3am), with more REM and light stage sleep happening in the second half of the night (3am to 7am). This means that people who go to bed by 11pm are experiencing more restorative, healthy sleep than those who stay up late into the night. Even if both people get the same number of hours of sleep!


While the complexities of sleep still contain a significant amount of mystery, professionals still tend to agree that getting through the sleep cycles properly throughout the night is critical for your health and well-being. If you can’t seem to sleep well at night, practicing good sleep hygiene, going to bed earlier in the evening, and talking to your doctor may help you with finding ways to overcome your sleeping difficulties.

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