Prior to the early 1920s, scientists believed sleep was passive — a state of low activity when a person's brain and body shut down for the night to rest and recover. Our understanding of sleep hadn't evolved much beyond the beliefs of the ancient Greeks, who viewed Hypnos, the god of sleep, as the brother of Thanatos, the god of death.
The scientific study of sleep had a breakthrough year in 1953 when Eugene Aserinsky and Nathaniel Kleitman of the University of Chicago observed eye movements during sleep and discovered two distinct forms of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. Through their research, the duo found that REM sleep was correlated with dreaming and increased brain activity (interestingly, closer to that of someone awake rather than unconscious). (Source)
By contrast, NREM sleep is characterized by deep, slow-wave activity and has itself been broken into three distinct stages: N1, N2, and N3. As we fall asleep and progress through the sleep stages, our brain waves progressively slow and increase in amplitude. (Source)
We now know that our bodies alternate between both REM and NREM sleep as part of the sleep cycle, and that the differences between those states are as profound as the difference between sleep and wakefulness.
Today, we're uncovering more about the deepest stage of sleep (stage N3), also known as the reason why you might experience a “sleep hangover” if you oversleep your afternoon nap alarm.
What Is Slow-Wave Sleep?
The Majority of Total Sleep Time Is NREM
Although both REM and NREM are important for different functions in our bodies, NREM sleep typically comprises 75% to 80% of total sleep each night. An average 8-hour night of sleep contains around five of these NREM–REM sleep cycles, each lasting about 90 minutes. Within that, slow-wave sleep (SWS) occurs mostly in the first third of the night and constitutes 10% to 20% of total sleep duration. (Source)
Slow-Wave Sleep Is When Your Body Repairs and Renews Itself
To understand why N3 is important, think of your body as a factory that performs several vital functions. As you drift off to sleep, your body begins its night-shift work — performing maintenance and repair all the way down to the cellular level. At the beginning of the N3 stage, the pituitary gland releases human growth hormone (HGH), which stimulates tissue growth and muscle repair. During this regenerative stage, the body repairs and replenishes tissue, builds bone and muscle, strengthens the immune system, and flushes out built-up waste products. (Source, Source, Source)
Slow-Wave Sleep Is the Most Restorative Sleep Stage
N3 is considered to be the most restorative stage of sleep, and most important for waking up feeling refreshed. This may be why the majority of N3 sleep occurs during the first half of the sleep period — if you don’t get the full amount of sleep you need, you’ll still get a good dose of N3. During N3 sleep brain activity declines to 75% below normal wakefulness level, which explains why you typically feel groggy and disoriented if you wake up during deep sleep — a phenomenon known as “sleep inertia.” (Source, Source)
Slow-Wave Sleep Declines With Age
The older you get, the less NREM sleep you'll have. People under age 30 typically have about two hours of restorative sleep every night, whereas adults over 65 might get only 30 minutes. N3 sleep continually decreases with age to the point that some elderly people may have no measured N3 sleep at night at all. (Source, Source)
What Does the Research Show About Slow-Wave Sleep?
Slow-Wave Sleep Acts as a Natural Anti-Anxiety Agent
University of California, Berkeley researchers showed that people who received more slow-wave NREM sleep had lower anxiety levels compared to those who had a sleepless night. Their study showcased the importance of NREM sleep specifically and its role in calming an overanxious brain. Interestingly, studies have shown that increased cortisol levels are correlated with longer periods of slow-wave sleep and shorter periods of REM sleep. (Source, Source)
Slow-Wave Sleep Plays a Key Role in Immune Function
Studies have shown that when fighting off an illness, your body may increase the amount of time spent in NREM sleep while deprioritizing REM. This occurs for two key reasons:
- Our body needs more frequent repairs to recover when “at war.”
- The reduced energy demand of NREM sleep leaves more fuel for maintaining a fever, which is an energy-intensive process.
Slow-Wave Sleep Is Important for Memory Consolidation
Deep sleep is critical for “sleep-dependent memory processing,” during which we form and encode new memories. A lack of slow-wave sleep negatively impacts the ability to perform memory tasks and the formation of declarative memory, which includes common knowledge and information about everyday events. (Source)
Studies show that lack of slow-wave sleep, increased sleep-wake disturbances, and prolonged periods of wakefulness can increase levels of amyloid plaques in the brain. Amyloid plaques, protein structures that form in the spaces between nerve cells, may exacerbate and accelerate the onset of Alzheimer’s disease.
Further studies suggest Alzheimer's disease disrupts the sleep-wake cycle, causing increased sleep fragmentation and decreased slow-wave sleep. Due to the reciprocal relationship between sleep and Alzheimer’s, a restful night’s sleep is imperative for memory consolidation and overall cognitive health. (Source)
You Can Make Up for Lost N3 Sleep
Sleep deprivation studies have consistently demonstrated that you can compensate for slow-wave sleep if you don't get enough. So if on an average night you typically have about 100 minutes of slow-wave sleep, but then miss a night of sleep, you will likely compensate by having around 200 minutes of slow-wave sleep on your recovery night. (Source)