Validation of Zulu Watch against Polysomnography and Actigraphy for On-Wrist Sleep-Wake Determination and Sleep-Depth Estimation

Abstract

Traditional measures of sleep or commercial wearables may not be ideal for use in operational environments. The Zulu watch is a commercial sleep-tracking device designed to collect longitudinal sleep data in real-world environments. Laboratory testing is the initial step towards validating a device for real-world sleep evaluation; therefore, the Zulu watch was tested against the gold-standard polysomnography (PSG) and actigraphy. Eight healthy, young adult participants wore a Zulu watch and Actiwatch simultaneously over a 3-day laboratory PSG sleep study. The accuracy, sensitivity, and specificity of epoch-by-epoch data were tested against PSG and actigraphy. Sleep summary statistics were compared using paired samples t-tests, intraclass correlation coefficients, and Bland–Altman plots. Compared with either PSG or actigraphy, both the accuracy and sensitivity for Zulu watch sleep-wake determination were >90%, while the specificity was low (~26% vs. PSG, ~33% vs. actigraphy). The accuracy for sleep scoring vs. PSG was ~87% for interrupted sleep, ~52% for light sleep, and ~49% for deep sleep. The Zulu watch showed mixed results but performed well in determining total sleep time, sleep efficiency, sleep onset, and final awakening in healthy adults compared with PSG or actigraphy. The next step will be to test the Zulu watch’s ability to evaluate sleep in industrial operations.

Sleep Depth and Patterns of In-Flight Sleep Behavior Across Time During Global Pandemic Conditions

“…However, actigraphy devices have a problem with low specificity [22][23][24], meaning that they are not very good at picking up awakenings during sleep intervals. The specificity of the Zulu watch to identify awakenings during a sleep interval compared to PSG under laboratory conditions is 26% [15]. Moreover, while sleep depth estimation by the Zulu watch has been tested against gold-standard PSG under laboratory conditions, no investigations of sleep depth or sleep architecture have ever been conducted during in-flight sleep.…”

“…Pilots were assigned the Zulu watch (Institutes for Behavior Resources, Inc. [15]) in May 2020 prior to COVID-19 support missions and wore the watches continuously until the completion of their mission (between May and July, 2020). Crews returned the watch to airline researchers directly upon returning to Brazil from their mission.…”

“…Brazil-based Azul Airlines, for example, estimated pilot fatigue using a biomathematical model prior to conducting 5 separate humanitarian missions to China between May and July of 2020 [14]. During missions, pilots wore a validated wrist actigraph (the Zulu watch, Institutes for Behavior Resources, Baltimore, MD [15]) and reported the sleep duration and quality of their in-flight rest periods using a sleep diary. Each mission consisted of 4 flight legs between 11-15 hours long each going from: 1) Brazil to a layover destination in Europe, 2) the layover destination to a destination in China to pick up COVID-19 relief supplies, 3) China to a return layover destination, and finally, 4) the return layover destination to the home airport in Brazil.…”

“…The Zulu watch is a commercial sleep tracker designed for use in operational environments which has been validated against PSG and actigraphy for sleep-wake determination and against PSG for the estimation of sleep depth [15]. Two-minute epochs within sleep events which are recorded by the Zulu watch are categorized as either interrupted sleep, light sleep, or deep sleep.…”

Pilot Sleep Behavior across Time during Ultra-Long-Range Flights

“…Brazil-based Azul Airlines, for example, estimated pilot fatigue using a biomathematical model prior to conducting five separate humanitarian missions to China between May and July of 2020 [20]. During missions, pilots wore a validated wrist actigraph (the Zulu watch, Institutes for Behavior Resources Inc., Baltimore, MD, USA [21]) and reported the sleep duration and quality of their in-flight rest periods using a sleep diary. Each mission consisted of four flight legs between 11-15 h long each going from: (1) Brazil to a layover destination in Europe, (2) the layover destination to a destination in China to pick up COVID-19 relief supplies, (3) China to a return layover destination, and finally, (4) the return layover destination to the home airport in Brazil.…”