Sleep and Health: It’s Not Just the Amount of Sleep but the Amount of Deep Sleep

image of woman in bed in deep sleep

Sleep is fundamental to good health and many people don’t get enough of it. In fact, the number of hours spent sleeping has steadily declined over the years. In the 1940s, the average person slept a full hour longer than we do today. The incidence of many chronic diseases is also on the rise and you might wonder whether changes in sleep patterns play a role in the epidemic of chronic disease we’re currently facing. Sleeping less than 6 to 7 hours per night is associated with a higher risk of type 2 diabetes, heart disease, and obesity. Plus, we simply don’t feel our best and are least productive when we skimp on sleep.

Why Sleep Matters

Sleep might seem like a mysterious thing – why do we even drift into a state of unconsciousness and stay in that state for hours? It must be important since we spend a full third of our life sleeping – but why? For one, sleep helps us consolidate the information that we learn through the day into long-term storage. But, that’s not the only function of sleep.

The brain has a voracious demand for nutrients and oxygen due to its high metabolic demands. But, as it uses those nutrients to create energy, waste products build up. What’s interesting about the brain is, unlike the rest of the body, it has no lymphatics, in the traditional sense, to carry away waste products. Instead, it uses a system called the G-lymphatics, a waste clearance system that ferries waste products out of the brain via cerebrospinal fluid. These waste products include damaged and misfolded proteins that can build up and contribute to Alzheimer’s and brain aging.

The G-lymphatic system is essentially a clean-up system for the brain and one that may protect against premature brain aging. What’s more, this clearance system is most active during sleep. Now, it’s not so hard to see why sleep is vital. If you don’t get enough of it, damaged proteins and toxins accumulate inside the brain.

Sleep Quality vs. Sleep Quantity

How well do YOU sleep at night? We’ve talked about sleep quantity – the importance of getting at least 7 hours of sleep nightly but what about sleep quality? Ultimately, it’s the amount of quality sleep that matters most. Tossing and turning and awakening exhausted is hardly beneficial and a growing number of people have this experience on a regular basis.

Why does sleep quality decline, especially as we age? Sleep actually occurs in stages: light sleep, REM sleep, and deep sleep. Light sleep, as the name implies, is the stage when you’re just entering sleep. During this phase, you can easily be awakened by noise or other forms of disturbance. Most adults spend roughly half of their sleep time in light sleep.

REM sleep, also known as rapid eye movement sleep, is so named because your eyes dart back and forth when you’re in it. REM accounts for 20 to 25% of total sleep and is the sleep phase where dreaming takes place. You typically enter REM sleep about 80 minutes after drifting off and cycle through it several times over the course of a night. If you don’t get enough REM sleep, you may have problems consolidating memories and remembering things that you learned during the day. So, REM is most important for memory and learning.

Then, there’s deep sleep. Deep sleep is, as the name suggests, a stage where you’re in a blissful state of unawareness. It’s very difficult to awaken a person from a deep sleep and when you wake them up, they’re extremely groggy and even a bit disoriented. Deep sleep is marked by delta waves, a type of wave you can see on an EEG tracing. By recording these waves in people of all ages, scientists know that delta waves decline with age, meaning people spend less time in deep sleep as you get older.

Why is deep sleep so important? Spending time in deep sleep is more restorative than time spent in light or REM sleep. If you don’t spend enough time in deep sleep, you’ll likely feel sleep deprived the next day, even if you spent the requisite time sleeping. Also, during deep sleep your brain releases growth hormone, the anabolic hormone that stimulates muscle and bone growth and cell and tissue repair. As with REM sleep, memory and learning consolidation also take place during deep sleep.

With deep sleep being important for tissue repair and memory, it’s concerning that we get less of it as we age. Scientists believe the age-related decline in deep sleep contributes to memory problems in older people. A young adult may spend 20% of their night in deep sleep, but the percentage drops to as low as 5% by age 80. That means older people get less restorative sleep, reduced release of growth hormone, and less efficient tissue repair.

How Can You Increase Deep Sleep?

Unfortunately, there isn’t a magical way to turn on deep sleep. However, researchers recently discovered that playing certain sound waves during sleep helps to enhance it. Still, this technology is not widely available yet. Are there simpler ways to boost the time you spend in a deep slumber? There’s some evidence that raising body temperature through intense exercise or sitting in a sauna augments deep sleep. Aerobic exercise, in general, seems to improve sleep quality. A study found that sleep quality was improved most by exercising in the morning.

The Bottom Line

Ultimately, sleep quality matters more than sleep quantity, at least to a certain point, and deep sleep declines as we age. We don’t have a sure-fire way yet to enhance the time spent in a deep sleep state, however, regular exercise may help. If you have persistent problems falling asleep, see your health care provider. Sometimes, undiagnosed sleep disorders or sleep apnea are a factor.



Neurochem Res. 2015 Dec;40(12):2583-99. doi: 10.1007/s11064-015-1581-6. Epub 2015 May 7.
NIH. “What is REM Sleep?”
Psychology Today. “The Mysterious Benefits of Deep Sleep”
American Sleep Association. “Deep Sleep”
Northwestern.edu. “Sound waves enhance deep sleep and memory”
Vasc Health Risk Manag. 2014; 10: 691–698.


Related Articles by Cathe:

Is It Normal to Have Problems Sleeping as You Get Older?

What is the Ideal Amount of Sleep for Good Health?

How Lack of Quality Sleep Limits Muscle Growth

How Much Sleep Do You Really Need for Health?


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