Although things are starting to change as modern science learns more about sleep, for the most part, sleep doesn’t truly get the respect it deserves, despite being an essential foundation of health and well-being. It’s not just the amount of sleep that matters, but also the quality of sleep; how well a person moves through the sleep stages throughout their sleep period.

Important processes that are critical to health and healing happen during deep sleep, the slow-wave phase of the sleep cycle. However, many people routinely don’t get enough sleep, missing out on essential slow-wave sleep time. Indeed, health experts throughout the world point to epidemic levels of sleep deprivation and sleep disorders.

Deep Sleep Explained

Each night, we cycle through four sleep stages: three non-rapid eye movement stages and one rapid eye movement stage (REM). The first sleep stage of the sleep cycle is a transitional non-REM stage. During this stage, breathing slows, heart rate decreases, muscles begin to relax and brain waves start to slow, producing the lightest sleep in the sleep cycle. This brief stage is just a few minutes long.

Breathing, heart rate and brain waves continue to slow during the second stage, though there are occasional bursts of brain activity, or spindles. Muscle relaxation deepens. Body temperature decreases. Eye movement ceases. Most of our sleep time is spent in this stage.

The high-voltage slow-wave sleep commonly referred to as deep sleep is the third phase of sleep and the final non-REM stage, accounting for about 13 to 23 percent of total sleep time. Respiration and heart rates reach their lowest points and brain waves slow further still. During this deep, restorative sleep phase, there’s a lot going on in the body. It is during this phase that toxins, including those associated with Alzheimer’s disease and dementia, are flushed from the brain.

Tissue growth and repair, immune system strengthening and hormone balancing also take place during this sleep phase. This impacts metabolic processes, including glucose management, and cellular repair and health, among numerous other health-impacting processes. The rest and restoration of the brain during slow-wave sleep is essential to optimal cognitive performance, impacting both learning and memory.

Slow-wave sleep is followed by REM sleep, the fourth sleep stage. During this stage, characterized by rapid eye movement and brain wave, respiration and heart rate patterns that are similar to those experienced while awake, we are likely to dream vividly. The amount of time we spend in each of those stages changes as we move through the entire cycle again and again, with the bulk of our deep sleep occurring in the earlier cycles and the proportion of REM sleep increasing in later cycles. People typically complete four or five sleep cycles in a night, assuming sleep isn’t disrupted, and that people are getting between seven and nine hours of sleep.

Slow-Wave Sleep Helps Protect Brain and Body

Insufficient and poor-quality sleep have long been associated with increased disease risk, including cardiovascular diseases, metabolic disorders, obesity, type two diabetes, certain types of cancer and more. Slow-wave sleep plays a major role in the maintenance and function of the systems that protect the body from disease, like the immune system, and those responsible for the very mechanics of how the body works, such as the interplay of hormones and the metabolic system.

Deep work, such as cellular repair and tissue growth, takes place during deep sleep. Failure to get enough of this deep, slow-wave sleep is disruptive to countless complex processes in the body and brain and can leave you more vulnerable to developing chronic health conditions and even disease.

How Much is Enough?

According to the Centers for Disease Control and Prevention (CDC), adults need between seven and nine hours of sleep nightly. When all is as it should be, then, that translates into 90 to 120 minutes of deep, slow-wave sleep per night.

To increase your chances of getting that deep sleep that is so essential to optimal cognitive and physical health, you need to plan your sleep with sufficient time to move through the sleep stages smoothly to complete enough sleep cycles each night.

Improve Your Deep Sleep

Set aside enough time each night for both a relaxing nightly routine in preparation for a good night’s sleep and the seven-to-nine hours you need to complete four or five sleep cycles. Go to bed and get up at the same times every day. Consider adding a warm bath or some meditation to your evening routine.

Get morning light exposure daily and some vigorous physical activity earlier in the day. Eat a healthy diet, avoiding ultra-processed foods and reducing simple carbohydrates. Increase natural fiber intake and consume the bulk of calories earlier in the day, with a lighter evening meal of lean protein and vegetables.

Don’t consume caffeine after early afternoon. Avoid blue light emitting devices, such as mobile phones and tablets, in the two to three hours before bedtime, as blue light can suppress melatonin production and delay sleep.