What is deep sleep
Sleep can be broken down into different ‘stages’ of sleep. Most commonly, sleep is divided into rapid eye movement sleep (REM) and non-REM (NREM) sleep. NREM sleep accounts for the majority of sleep (75-80% of total sleep duration), while REM sleep makes up the rest. Within NREM sleep, there are four 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 has been shown to be more important than total sleep time in affecting physiological 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, among others.
By measuring these changes using wearable technologies (accelerometers and photoplethysmography [PPG]), a close approximation of sleep stage can be made. Using 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 with total sleep time, the restorative 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, compared to 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, affecting the duration of deep sleep, has been linked to many negative health outcomes, including cardiovascular disease, diabetes, neurodegenerative diseases, poor cognitive function, and many more conditions. As such, it is important to get adequate amounts of sleep that is of good quality, permitting deep sleep.
Normal or acceptable range
Currently, there are not widely accepted values specific to deep sleep. There appears to be an age-related decline in the duration of deep sleep, with minimal change in other sleep stages, in absolute terms. Of each sleep session, most individuals have 13-23% of their duration in deep sleep. The recommended amount of deep sleep has not fully been evaluated, but many experts believe that it is better to have more than less, although high amounts of deep sleep may indicate short term deficiencies.
While it is relatively easy to approximate total sleep time by tracking sleep and wake times, deep sleep is much harder to quantify. Use of EEG devices provide a strong understanding of sleep stages and progressions but are less obtainable for an individual on a regular basis. However, using accelerometers and PPG wearables, such as Biostrap, 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 physiological variables, cognitive and/or athletic performance, mood, fatigue, and other health-related outcomes. As a variable that is more challenging 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.