Sleep Measurement Study

Sleep behaviour has critical importance to health and wellbeing. Insufficient sleep duration and poor sleep quality are independent contributors to high blood pressure and cardiovascular disease, depression, obesity and diabetes. Given the importance of sleep duration and quality to health, the importance of accurately monitoring sleep duration and quality in everyday clinical practice is becoming more evident.

The 'gold standard' physiological measure of sleep is sleep polysomnography (PSG). PSG is used to quantify measures of sleep, including length of sleep, time taken to fall asleep, sleep efficiency. The disadvantages of sleep PSG include the need to attend a laboratory, use of expensive equipment, specialised staff to administer PSG, and to score and interpret the PSG outputs, which limit its use in larger, or free-living studies.

Another method to quantify indices of sleep is based on actigraphy, demonstrating 90% agreement with polysomnography. Wrist actigraphy allows sleep assessment over several days and measures daily sleep-wake cycles. However, the data is in the form of manufacturer-specific activity 'counts' over a specific time window, making it difficult to compare the data with different accelerometer brands. Recently wrist-worn accelerometers have become increasingly used for objective measurement of physical activity in large population studies where participants are often asked to wear them for 24 hours continuously, to maximise compliance. These devices therefore collect data that could be used to estimate sleep parameters, however to be able to use, pool or compare these data there is a need for sleep algorithms that can be applied to datasets from different accelerometer brands. The latest generation of accelerometers measure acceleration in universal units improving comparability among different brands of accelerometers and allowing more control in the data processing. Moreover, now there is a sleep detection algorithm that can applied to data from different raw-data accelerometer brands and is freely available as a part of GGIR package in R (software environment for statistical computing and graphics). The three widely used raw-data accelerometer brands are the Axivity, ActiGraph and GENEActiv and ActivPAL which is a thigh-worn accelerometer that provides a measure of posture using proprietary algorithms; however, raw data are now available.

Studies that have validated the accelerometers with the PSG produced mixed results which can be attributed to use of manufacturer specific sleep algorithms and different accelerometer placement (dominant vs. non-dominant wrist vs. hip). Therefore validation of a sleep algorithm that can be applied to different accelerometer brands against PSG warrants investigation.