Deep Sleep: Why It's the Foundation of Full-Body Restoration (And How to Get More of It)

Written by Doug Hay — Medically Reviewed by Dr. Joel Kahn, MD — Published December 11th, 2024

What if the most important part of your day happens while you sleep?

Most of us view sleep as passive — where your brain and body shuts down to rest.

But it's actually the opposite. Sleep is your body's essential period of repair, restoration, and rejuvenation.

Arguably, it's the most critical part of your day... not just energy, but immunity, cognitive function, and longevity.

And while all stages of sleep serve a vital role, none are more important than deep sleep.

So what happens when the lights go out, and how can you help maximize your quality deep sleep?  

Let's take a look.

The 4 Stages of Sleep

While you may be in bed for 6-9 hours, your body is actively working through various stages of sleep every 10-45 minutes.

Each stage serves its own purpose, forming a sleep cycle, lasting 90-110 minutes.

You can split the stages into two types: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep.

Non-Rapid Eye Movement (NREM)

• Stage 1, Falling Asleep: This stage only lasts a few minutes (5% of sleep). You're asleep, but your muscle tone and breathing remain consistent to an awake state.
• Stage 2, Light Sleep: You're in this stage more than any other — about 45% of your sleep — where your body is preparing for deep sleep.
• State 3, Deep Sleep: It's in this stage when your body repairs muscles, boosts your immune system, and even helps you grow new tissue. You're in Deep Sleep for about 25% of your sleep.

Rapid Eye Movement (REM)

• REM Sleep: REM is known as the dream stage, and you're in this stage about 25% of the time. REM is where your brain processes new information and emotions, and locks in new memories.

The time spent in each stage varies throughout the night, with more REM sleep happening in the second half of the night.

What Happens When You’re Woken Up Mid-Sleep Cycle?

While your body is really good at efficiently working through a sleep cycle, life isn't always that tidy and sometimes you're abruptly woken up mid-cycle. (We see you parents!, parents.)

Getting woken up in the middle of a sleep cycle can leave you feeling groggy, disoriented, and moody.

When you’re woken during deep sleep, you may experience sleep inertia, that heavy, sluggish feeling that makes it hard to fully wake up. This happens because your brain hasn’t transitioned into a lighter stage of sleep.

Interruptions during REM sleep, where dreaming occurs, can leave you mentally foggy.

Over time, frequent interruptions can result in poor focus, irritability, and even long-term health issues like weakened immunity and hormonal imbalances.

What Makes Deep Sleep so Important?

If you've tracked your sleep using a smart watch or ring, you may have developed an obsession with deep sleep.

You can be in bed for hours, but if you aren't dropping into deep sleep, your "sleep score" will suffer greatly.

And there's science to back this up. Deep sleep is when your body gets to work on vital restoration tasks that you can’t perform while awake. Here’s what’s happening during each of those 10-15 minute cycles of deep sleep:

1. Cellular Repair and Growth

During deep sleep, your body releases growth hormones, which helps repair tissues and muscles that were damaged during the day.

This phase is essential for muscle recovery, wound healing, and cellular regeneration.

2. Immune System Support

Deep sleep strengthens your immune system by promoting the production of cytokines, proteins that help fight infections and inflammation.

Good quality sleep is associated with a reduced risk of chronic diseases and a significant risk of developing sickness from common colds and viruses.

3. Energy Restoration

Your body replenishes energy stores and restores glycogen levels, fueling your cells for the next day.

This process helps you wake up feeling refreshed and ready to tackle the day.

4. Brain Detoxification

The brain’s glymphatic system becomes highly active during deep sleep, flushing out waste products like beta-amyloid, a toxin linked to Alzheimer’s disease. This nightly cleanup is crucial for long-term brain health.

Simply put, people who get enough deep sleep are less likely to develop chronic conditions like diabetes, heart disease, and neurodegenerative disorders. Improving your deep sleep is a long-term investment in your overall health.

How to Maximize Deep Sleep

For adults, deep sleep typically accounts for 13-25% of your total sleep. That’s about 1.5-2 hours each night if you’re sleeping for 7-9 hours, and it varies based on factors like age and activity level.

So with deep sleep being so important, how can you help maximize it?

• Stick to a consistent sleep schedule... yes, even on weekends.
• Exercise regularly, but avoid intense workout within an hour or two of bedtime. Even just 10 minutes of daily exercise has been shown to improve sleep quality.
• Create a dark, cool, and quiet sleeping environment to reduce mid-cycle wake-ups. The ideal room temperature is around 65-68 degrees.
• Prioritize evening relaxation, like warm baths, evening tea, or a cozy reading session.
• Limit alcohol and caffeine intake, especially in the evening. Caffeine has a half-life of up to 6 hours (meaning half of the caffeine from a noon coffee may still be in your system at 6 pm), and alcohol -- while it  may help you feel relaxed and fall asleep more quickly -- is shown in multiple studies to disrupt sleep patterns and limit REM sleep.
• Stop eating at least an hour before bed to allow your food to digest and move into your small intestine. Most nutritionists recommend you stop eating at least 2-3 hours before sleep.
• Optimize your diet with sleep-supporting nutrients like magnesium and L-Theanine.

Struggling to Get Enough Sleep? Don't Stress

Deep sleep is the foundation for physical restoration, mental clarity, and emotional balance, and yet, many of us struggle to get the recommended amount. 

Instead of stressing about it — which can lead to even worse sleep — try adopting one new habit at a time.

Maybe the first is not eating right before sleep or cutting out screens once in bad. Or maybe it's trying a new nighttime tea or magnesium supplement.

By starting with one simple change, you'll begin to see what's working without the stress of overhauling your entire routine.

And eventually, start giving your body and mind the best chance to recharge and thrive.