What is Deep Sleep?

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It seems that the more we study sleep and how it affects the body, the more we realize that it is a profoundly complex biological process. Sleep involves all aspects of the body and can be intensely variable. While we tend to think of someone as either “asleep” or “awake,” the truth is that what it means to be asleep is extremely complicated.

One example of this is the stages that make up the sleep cycle. Studies that analyze brain activity in sleeping people have identified that we move through several different phases of sleeping. These are generally identified as stages 1-4 and then REM (rapid eye movement) sleep. Stages 1-4 are also sometimes called NREM (non-REM) sleep.

Deep sleep is defined as stages 3-4 of NREM sleep, and it has some unique and important characteristics. In this article, we’ll provide more detail about what deep sleep is, why it matters, what affects it, and how you can try to get more of it.

Does Deep Sleep Have a Specific Meaning?

Colloquially, we use the term “deep” to refer to any sleep that feels profound or heavy. But the specific meaning of deep sleep refers to stages 3 and 4 of the sleep cycle. Even though it’s normal to talk about sleeping deeply in general society, in this article, we’ll focus on this more particular meaning of “deep sleep.”

In stages 1 and 2 of the sleep cycle, we start to doze off and settle into sleep. In these stages, you still have more involved breathing and brain activity even though your body is beginning to slow these things down. For this reason, it is still referred to as fast-wave sleep.

In stages 3 and 4, your body moves into slow-wave sleep. This means that brain waves start to slow way down, including with delta waves, the slowest that the body produces. Virtually all systems of the body, including the heart and lungs, seem to slow down and rest at this point. It is much harder to be disturbed or woken up (such as from light or noise) when you are in deep sleep.

Towards the end of stage 4, the body begins to move into REM sleep. During REM sleep, brain activity picks up significantly. Though dreaming can happen in all stages, the most intense dream activity usually occurs during REM sleep. Because the body is ramping up from the slow down in stage 4 to the activity of REM sleep, the latter part of deep sleep is when many people, including children, experience sleep disturbances like nightmares, sleepwalking, night terrors, and bedwetting.

Why Does Deep Sleep Matter?

It is necessary to keep in mind that there are still many mysteries when it comes to sleep and understanding exactly how it affects our bodies and our physical and mental health. That said, from observing people in sleep studies, it appears that deep sleep plays an important role in how our body recovers and recuperates. For example, during stage 3 sleep, the body releases the bulk of its human growth hormone (HGH), which is involved in muscle repair. Deep sleep also gives the body a chance to slow down and become rejuvenated especially when compared with the REM sleep stage that follows it.

People who have interrupted sleep or otherwise do not get sufficient sleep to progress through multiple sleep cycles may miss out on these positive effects of deep sleep. Not only can this make someone feel drowsy, but it may also have spillover effects to broader physical and mental well-being.

How Does Age Affect Deep Sleep?

As we age, the patterns of our sleep are prone to change. Some of this can be a natural result of modifications to our Circadian Rhythm. For example, there is a tendency for the elderly to have a greater percentage of fast-wave sleep compared to younger people. In addition, other health conditions can interfere with consistent sleep and normal progression through sleep cycles. These conditions can range from frequent urination to anxiety to sleep apnea to arthritic pain to cardiovascular or pulmonary problems.

Unfortunately, a wide range of health issues that are more common in older people can disturb sleep, limiting time in deep sleep and REM sleep. In addition, our bodies produce less HGH as we get older, so even when in deep sleep, elderly people do not get as much restorative effect from this hormone.

How Can You Get More Deep Sleep?

There is no simple answer about how to get more deep sleep. So far, there’s no known way to trigger the body to move between sleep stages; instead, this has to happen naturally. As a result, the best approach to getting more deep sleep is to improve the quality and quantity of your sleep overall.

A big part of this is to improve your sleep hygiene, which is the term that is used to refer to your sleep environment and habits around sleep. Normalizing the times that you go to bed and wake up, minimizing potential disruptions to your sleep, creating a comfortable sleep environment, and other steps can all boost the chances that you will be able to get to sleep quickly and stay asleep. This allows your body to enter the early stages of sleep and move organically through deep sleep, into REM sleep, and then back again. For more specific guidance, check out our article on 15 Tips to Improve Sleep Hygiene.

Beyond focusing on sleep hygiene, remember that you can talk with your doctor if you are having chronic issues sleeping or daytime sleepiness. In some cases, the underlying cause of these problems can be addressed to help pave the way for you to get better sleep, including deep sleep. Talking with a doctor is also a good idea before starting to use any medicines or supplements to try to help you sleep.

While some of those may help in the short-term, most aren’t intended or tested for long-term use, and their impact on sleep stages is often unclear. For this reason, consulting with a health professional is your best bet for getting your sleep back on track.

Is Sound Sleep the Same as Deep Sleep?

Yes and no. Technically speaking, there is no real definition for “sound” sleep. It’s a general term that is used casually to refer to a person’s general feeling about how well they slept. It’s possible for someone to have felt that they slept “soundly” without having actually spent a great deal of time in deep sleep. Many factors can influence whether a person subjectively feels that they slept soundly, but they aren’t basing that evaluation on data analyzing their brain waves or sleep activity.

Deep sleep, on the other hand, is a specific term. In all likelihood, people who get solid deep sleep and progress smoothly through sleep cycles will be more likely to feel as though they slept “soundly,” but the fact is that we don’t have any scientific data to know that this is true. This doesn’t mean that there’s anything wrong with saying you slept soundly, but just know that it doesn’t have the particular meaning associated with deep sleep.


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