Deep sleep is known to be the body’s most rejuvenating sleep stage. But research suggests that it’s every bit the brain tonic, too.
Compared to lighter sleep and REM, deep sleep seems like a dormant phase – your breathing, brain activity, and blood pressure all decrease. Even your temperature reaches a daily low.
During this lull, though, your body’s regenerative potential skyrockets.
Scientists believe that deep sleep (also called slow-wave sleep) is so restorative in part because of the ramped-up ‘rest and digest’ nervous system activity. This coincides with a peak in Growth Hormone levels, which promotes tissue repair. 
You’ll typically spend 60 – 90 minutes in deep sleep each night. But the amount of slow-wave sleep declines dramatically with age – and that decline can predict a variety of health issues.
Why Is Deep Sleep Necessary?
To understand how important deep sleep is for your health, first you have to appreciate just how dangerous it used to be.
Humans have an unusually high arousal threshold during deep sleep. (This means that even loud sounds can’t wake us.) So, it follows that any pre-modern person sleeping in the savanna would have been extremely vulnerable to attacks in their most profound snoozing state… They had to cheat death each night!
If our lives were literally on the line each time we plunged into deep sleep, then deep sleep had to be essential. And research shows that it is.
Metabolic diseases (including high blood pressure and diabetes) are usually associated with aging. But research has shown that even healthy young people will have metabolic impairment similar to seniors when deep sleep is disturbed.
There are several reasons why deep sleep is so good for the metabolism. Growth hormone, for instance, which is elevated in deep sleep, is known to increase insulin sensitivity and muscle mass.[9, 10] Both of these actions stabilize blood sugar and limit fat gain. Slow-wave sleep also limits cortisol production, which positively impacts insulin levels and blood pressure.[11, 12]
Many have questioned why Mother Nature, in her infinite wisdom, would make us so oblivious to the outside world while we’re deep sleeping. Well, it turns out that deep sleep is bath time for the brain. (You wouldn’t want to be interrupted either.)
In all the 100 billion-or-so neurons firing off all day, your brain creates a substantial amount of waste products, including beta amyloid and tau. These sticky proteins accumulate in the brain and can eventually cause neurodegenerative diseases such as Alzheimer’s.
Now, when you’re in the thick of deep sleep, the spine pumps cerebrospinal fluid (CSF) up through the brain stem, effectively bathing your brain and cleaning out the toxic proteins via the glymphatic system. (Who knew?)
Scientists have even correlated the long, slow waves generated by your brain in deep sleep to these pulses of CSF. 
Apart from the obvious memory-boost that comes with a squeaky-clean brain, the slow waves of deep sleep are thought to retrieve memories from a day and tuck them into long-term storage, freeing up RAM for the next day’s events.
In a 2009 research paper for the Journal of Clinical Sleep Medicine, sleep scientist Matthew Walker noted that slow-wave sleep helped to consolidate memories and improve the recall of facts and events. Other studies have shown that losing sleep functionally impairs the hippocampus – the brain region responsible for short-term recall.
How to Get More Deep Sleep
Even if you do sleep the recommended 7-8 hours, you still might wake up feeling unrefreshed. That can be a sign that you’re missing out on deep sleep.
Here are five science-backed tips to make sure your body’s not skimping on the slow waves:
1 – Eat more fiber.
Eating healthy is a no-brainer for better sleep. (You want to get enough of the right minerals and proteins to support proper brain chemistry.) But it turns out that fiber, humble fiber, is the deep sleep whisperer.
Even though we don’t know exactly why it works, fiber by itself has been shown to increase slow-wave activity and duration. Diets high in saturated fat and carbs, on the other hand, can lead to lighter sleep. 
You can start off by eating more whole-grains and leafy greens and supplementing with fibers like psyllium husk or resistant starch.
2 – Try acoustic stimulation.
Using sounds to influence sleep is as old as sleep research itself. Yet it’s only been in the last decade that ‘acoustic stimulation’ has been reliably used to increase deep sleep.
Scientists found that by matching the frequency and pulses generated in deep sleep, we can bolster our slow-wave sleep activity.  But you don’t need to be hooked up to an EEG at a sleep lab to get the benefits of acoustic stimulation anymore.
There are a few sleep headbands that measure your brain waves and emit slow-wave frequencies to try to enhance deep sleep when it’s happening.
3 – Learn something.
Deep sleep starts off as a large proportion of sleep in children, then gradually fades to 13-23% of sleep in adults.
Even though researchers haven’t correlated this decrease in slow-wave activity to less learning, per se, there’s evidence that learning a new task can promote deep sleep.19 Deep sleep also improves whatever skill you learn, too.  So it’s a win-win.
Studies have focused mostly on physical problem solving, but any kind of learning (words, math, facts) could be beneficial.
4 – Keep your skin warm.
While cooler climates are essential for sleep, even a .4 degree increase in skin temperature can lead to more deep sleep and fewer awakenings, given that your core temp doesn’t increase too.
Researchers have used high-tech thermosuits to achieve optimal sleep through skin warming. But a practical way for you to get deeper sleep this way is simply to wear warm socks in a cool bedroom. (Breathable/comfy sheets and comforters help, too.)
5 – Black out your room and wear a sleep mask.
Any type of blue-light exposure is a potential deep sleep disruptor – the light creeping in through your blinds, even small indicator lights on your electronics.
Many biohackers and regular people alike have found success using blackout shades and placing electrical tape over blue-light sources in a sleep space. But if you’d rather take a small step first, eye masks are a great option.
For now, deep sleep is the most popular sleep stage. It’s the one that makes us feel ready to take on the day; it clears our minds and enhances memory. But sleep scientists caution us from focusing too much on any one phase, because each stage has an important role to play.
That said, people over forty. and anyone struggling with cognitive decline. should be as proactive about deep sleep as possible. Just make sure to consult your doctor before attempting any sleep intervention.
About The Author: Dan Dowling is a health and fitness expert based in Albuquerque, NM. A biohacker-turned-writer, Dan cured two years of insomnia with time-restricted eating and beat chronic fatigue with essential amino acids and B3.