Get to know the four sleep phases

The discussion around a good night’s sleep generally focuses on how many hours we should spend in bed every day, but this is not the only criteria we should consider. The quality and continuity of sleep phases are as important as the number of hours a person sleeps.

As our body goes through a cycle that lasts around 24 hours every day, the circadian rhythm, our sleep is also divided in cycles that are repeated through the night.

Sleep cycles are divided in different stages (or phases) with specific functions. In short: the sleep phases make up a cycle; the repetition of this cycle makes up the act of sleeping; and the act of sleeping is part of our great circadian cycle.

What are sleep cycles

Sleep is divided in four phases that, together, make up a sleep cycle.

Throughout the night, the body repeats this cycle four to six times, each lasting around 90 minutes. This is why, ideally, you should sleep for around six to nine hours.

These cycles are not identical. During the night, sleep phases get shorter or longer, and may even not appear in certain cycles. An example of this is the REM phase. In a ‘normal’ sleep, this phase only appears after sleeping for one hour and a half, which can happen on the night’s second cycle.

But the duration of each sleep cycle can also be influenced by outer factors: time, temperature, noise, light and feeding. Caffeine, for example, may diminish non-REM sleep duration, leading to dramatic consequences in the next day.

Understanding sleep phases

Now, you’ll get to know all sleep phases, what happens in your body and brain during each phase, and how long they last.

Phase 1: Non-REM N1

Average duration: between 1 and 5 minutes, accounting for around 5% of the sleep cycle.

Being the first phase in the sleep cycle, Phase 1 is the shorter of them all. Since the brain is still in between being awake and asleep, it is the phase in which we are more prone to waking up.

Since it represents a transition moment and the body is still not fully relaxed, we are still conscious about what’s going on around us, and sometimes we don’t even realize we are starting to sleep.

Some physical characteristics of Phase 1 are:

– Breathing slows down

– Arterial blood pressure drops

– Heartbeat becomes constant

– Body temperature decreases

– Alpha and theta brainwaves are produced

– Muscles relax

– Muscles may contract

People sleeping in a more regular way tend to spend less time in Phase 1 during the latter night cycles.

Phase 2: Non-REM N2

Average duration: Around 25 minutes in the first cycle, and increasing in the remaining cycles. It can reach up to 50% of the total sleep duration.

As happens with the previous phase, Phase 2 is considered a light kind of sleep that starts preparing the body for the deeper sleep stages. It is much harder to wake up a person during Phase 2 than during Phase 1, since the person is less receptive to external stimuli.

During the second sleep phase:

– muscles are relaxed

– the heart and breathing become even slower

– the body’s metabolic processes slow down

– the brain produces high-frequency wave shots called sleep spindles.

Phase 3: Non-REM N3 or deep sleep

Average duration: Between 20 and 40 minutes in the first cycles, decreasing in the final cycles.

Now we are at the deep sleep stage, when it’s even harder to wake up. By the way, it is not recommended to wake someone up during Phase 3, since this may cause what’s called sleep inertia, that feeling of disorientation that many people experience when they wake up.

Cognitive tests performed in people woken up during N3 show that they had damaged brain activity for up to an hour.

Phase 3 can also be called Delta Sleep or Slow Wave Sleep, both referring to the type of waves emitted by the brain during this stage.

Among the many functions and benefits of Deep Sleep, we can find:

– Muscle and brain restoration

– Real brain and mind rest and relaxation

– Restoration and reconstruction of tissues, bones and muscles

– Immunity strengthening

– Creativity development

– Memory processing

– Release of GH, the growth hormone

Alcohol consumption, stress, sleep apnea and diseases such as fibromyalgia may hamper or even prevent a person to reach the deeper sleep phases.

Phase 4: REM sleep

Average duration: Unlike other sleep phases, that become shorter as the night goes by, REM sleep lasts more. During the first cycle, it lasts for around 10 minutes, and it can last up to an hour as sleep progresses.

The last phase of the cycle is the REM sleep – REM meaning Rapid Eye Movement, exactly what happens. During this phase, the body goes through what we call atony, a kind of temporary muscle shutdown. Only eyes and breathing muscles remain active.

Some characteristics of REM sleep are:

– Increase in brain activity, which was slowly diminishing its pace during the earlier sleep phases

– More vivid dreams, related to said increase in brain activity

– More erratic and irregular breathing

At the end of the REM sleep, a new cycle begins, but not necessarily at Phase 1. It is perfectly normal to start it from Phase 2, jumping the first ‘half awake, half asleep’ stage. Even better, or we would be waking up more times during the night.

Good night, and good sleep.