What is deep sleep
Sleep can be broken down into four different ‘stages’ of sleep. Most commonly, sleep is divided into rapid eye movement sleep (REM) and non-REM (NREM) sleep. NREM sleep accounts for most of the sleep (75-80% of total sleep duration), while REM sleep makes up the rest. Within NREM sleep, there are three stages; the first stage is light sleep and is mostly the transitory onset of sleep; the second stage is also considered light sleep but makes up a longer duration than stage 1.
The third and fourth stages are considered ‘deep sleep’ and are characterized by slow brain waves. Deep sleep makes up roughly 13-23% of nightly sleep. It is during these stages that sleep is restorative and leads to many adaptive physiological outcomes that help the body adapt and repair. As such, deep sleep is more important than total sleep time, affecting health outcomes.
How it is measured
Deep sleep is often identified by slow waveforms on an electroencephalogram (EEG), which measures brain wave activity. As an alternative, deep sleep has been shown to have decreased movement and altered vital signs, particularly: lower heart rate, higher heart rate variability, lower blood pressure, lower temperature, and decreased sympathetic activity.
By measuring these changes using wearable technologies (accelerometers and photoplethysmography [PPG]), a close approximation of sleep stage can be made. This technology allows for passive measurement with much less equipment than a traditional EEG or polysomnogram.
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Correlation with health conditions
Much like total sleep time, the therapeutic benefits of deep sleep have robust physiological effects across many organ systems. However, deep sleep appears to be a better indicator of the quality of sleep than the total duration of sleep.
Deep sleep has been shown to affect growth hormone production, glucose metabolism, synaptic processes (e.g. learning/memory formation), and immune function changes. Sleep restriction, influencing the duration of deep sleep, has been linked to many adverse health outcomes, including cardiovascular disease, diabetes, neurodegenerative diseases, poor cognitive function, and many more conditions. As such, it is essential to get adequate amounts of good quality sleep, permitting deep sleep.
Normal or acceptable range
Currently, there are not widely accepted values specific to deep sleep. For each sleep session, most individuals have 13-23% of their duration in deep sleep. The recommended amount of deep sleep has not been thoroughly evaluated, but many experts believe it is better to have more than less. It should be noted that exceptionally high amounts of deep sleep may indicate short-term deficiencies.
Deep sleep is a complex biometric that is difficult to quantify. EEG devices provide a strong understanding of sleep stages and progressions but are less realistic for an individual on a regular basis. However, using accelerometers and PPG wearables, light and deep sleep can be approximated on a nightly basis and easily tracked over time.
As with total sleep duration, tracking deep sleep can provide insight into its contribution to changes in health-related outcomes. As a more challenging variable to quantify, monitoring deep sleep over time can also provide insight into lifestyle changes and how they affect deep sleep. For example, tracking how a medication affects deep sleep may provide insight into its efficacy or side effects.