Deep Sleep
By Dr. Ryan Neely, Ph.D.
If you own a fitness or sleep tracking device, you may have noticed that your sleep report divides the course of your night into different sleep stages, including light sleep, REM, and deep sleep. The American Academy of Sleep Medicine (AASM) currently recognizes 4 distinct stages of sleep, termed N1, N2, N3, and REM. A healthy night of sleep involves multiple cycles through each of these stages, which have distinct physiological importance. However, N3 sleep - also known as “Deep Sleep” or “Slow Wave Sleep” - has received extra attention recently for the potential restorative role it may have for both brain and body. Here, we’ll focus on how N3 sleep is defined, and then dive into some of the unique features of this sleep stage that make it so important for maintaining healthy homeostasis.
Stage N3: Slow waves and slow hearts
Although sleep researchers and clinicians classify sleep into discrete stages, like many things in biology, the physiological changes that accompany various depths of sleep occur on a continuum. Regardless, one of the most prominent features of deep sleep is the presence of high amplitude delta waves and slow waves recorded by electroencephalogram (EEG). While some slow wave activity is present during other stages of sleep, during deep sleep these slow oscillations in the approximately 0.5 to 3 cycles/second range tend to dominate the EEG signal.
The above images show typical EEG patterns that can be observed during wakefulness (top) and slow-wave sleep (bottom), which is dominated by high amplitude slow oscillations.
The EEG measures large-scale electrical activity from the outside of the scalp, so the presence of slow waves signifies that the voltage of large populations of neurons are slowly fluctuating synchronously in time. Other features of stage N3 sleep include a slowing of the pulse and breathing rate as well as decreased muscle tone. Typically, sleepers spend the most time in this sleep stage earlier in the night, and tend to be less responsive to sensory stimulation like light or noise.
The Benefits of Sleeping Deeply
At face value, the value of deep sleep may seem elusive, not unlike the whole enterprise of sleep itself. After all, what could be the purpose of putting the body into a state of low responsiveness? What reason does the brain have for generating slow, coordinated fluctuations in voltage that we can observe as slow waves? The complete picture of what happens to the body and brain during slow wave sleep is still an ongoing area of investigation. However, there is convincing evidence for a number of important functions served by deep sleep.
Learning and memory: The role of sleep in enhancing learning and memory retention is well established, and deep sleep appears to be an important component of the memory consolidation process. The details of exactly how this occurs however are still up for debate. Some studies have suggested that deep sleep is important for improving declarative memory (i.e. memorizing facts), whereas REM sleep is better suited to promote the retention on non-declarative memories (such as how to ride a bike) (Plihal & Born, 1999; Rasch & Born, 2013). However, other research has shown this distinction is hard to generalize in other contexts. Another hypothesis that has received significant attention is the reactivation/consolidation model. To summarize a complex theory, this idea suggests that memories are “replayed” during sleep by reactivating neural patterns corresponding to the initial encoding of the memory that occurred during wakefulness. This reactivation corresponds to the transfer of information from short- and medium-term memory to brain regions where it is transferred to long-term memory storage. The slow oscillations that occur during deep sleep are thought to be important for choreographing this transfer process (Sirota & Buzsáki, 2005).
Immune function: Research has demonstrated several links between sleep and the activity of the immune system. During the early phases of sleep, levels of pro-inflammatory (i.e. immune-activating) circulating factors increase, such as the cytokines IL-1β, IL-6 TNFα, and IL-12. The inverse also appears to be true - injection of pro-inflammatory cytokines can enhance deep sleep (Kapsimalis et al., 2005). Taken together, it appears that deep sleep promotes activation of the adaptive immune system, at least in humans.
Hormone regulation: Deep sleep is thought to play a critical role in modulating the release of endocrine hormones, such as cortisol, epinephrine, prolactin, and growth hormone (GH) (Born & Fehm, 1998). Growth hormone in particular appears to be sleep-dependent, such that artificially delaying deep sleep impairs growth hormone production, and pharmacologically enhancing deep sleep can increase its release (Van Cauter et al., 1997).
Removal of waste products from the brain: One of the more interesting developments in the field of sleep research is the discovery that sleep, and specifically delta-wave sleep, is associated with clearance of waste metabolites from the brain (Xie et al., 2013). During periods of deep sleep (and also under anesthesia), there is a pronounced increase in the exchange between the interstitial fluid, which surrounds neurons and glia in the brain, and the cerebrospinal fluid which serves in part as a reservoir of interstitial fluid. This convective process has been shown to remove metabolic waste products from in and around neurons, including amyloid beta, which is known to be involved in the development of Alzheimer’s disease.
Conclusions
Although there is undoubtedly more to understand about the physiological importance of deep sleep, what we do know has shown it to support several functions that are critical for health. During deep sleep, slowing of the brain rhythms accompanies a slowing of the heart and breathing rate, and the sleeper becomes less responsive. During this time of dissociation from the outside world, the brain and body get to work. Memories are replayed and consolidated for efficient long-term recall, the immune system kicks into high gear, growth-promoting hormones are released, and the brain begins the process of cleaning out its waste products to preserve optimal functioning. These critical functions make it clear why health experts are continuing to recommend a focus on getting enough sleep, but also healthy sleep that allows the sleeper to cycle through each of the four stages, including deep sleep. Although it may seem like our brains are most hard at work during the day, they have a lot to do at night too!