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How does CentrePoint Calculate Sleep Periods?

Last Updated: Oct 23, 2017 08:52AM CDT

Overview

CentrePoint detects sleep periods by using a combination of either the Cole-Kripke or Sadeh algorithm (Cole-Kripke by default) and the Tudor-Locke algorithm. CentrePoint first applies the Cole Kripke Sleep Scoring Algorithm to determine which epochs (1 minute "chunks" of data) are marked as Sleep vs. which are marked as Wake, and then the Tudor-Locke method is applied to determine sleep periods. Each is explained below:

 

Cole-Kripke Sleep Scoring Algorithm

The Cole Kripke algorithm was derived from research performed by Roger Cole, and Daniel Kripke in the technical note Automatic Sleep/Wake Identification from Wrist Actigraphy. This paper is also available on ActiGraph's research database here. This algorithm is primarily used to score adult populations.

ActiGraph's Implementation of the Cole-Kripke Algorithm

First, we adjust the epoch data to help reduce the variation of the counts from the paper compared to our devices:

  1. Using the y-axis epoch data, scale the count values by 100.
  2. If any of those scaled values are over 300, set them to 300.

Once the counts have been adjusted, we determine if each epoch is sleep or awake based on a seven-minute window. It includes the four previous epochs and two epochs after the current.

Note: any missing epochs are considered ZERO. This happens if the current epoch is at the beginning or end of a dataset.

The window of data is put through the following algorithm:

.001 * (106 * Epochx-4 + 54 *Epochx-3 + 58 * Epochx-2 + 76 * Epochx-1 + 230 * Epochx+ 74 * Epochx+1+ 67 * Epochx+2); 

If the result of that algorithm is LESS than 1, then the current epoch is considered asleep.

 

Sadeh Sleep Scoring Algorithm

The Sadeh algorithm was derived from fundamental research performed by Avi Sadeh, Katherine Sharkey, and Mary Carskadon entitled Activity Based Sleep-Wake Identification: An Empirical Test of Methodological Issues. This paper is available on ActiGraph's research database here. This algorithm is primarily used for younger adolescents as most of the research was performed on children and young adults.

ActiGraph's Implementation of the Sadeh Algorithm

The Sadeh algorithm uses an 11-minute window that includes the five previous and five future epochs.

Note: any missing epochs are considered ZERO. This happens if the current epoch is at the beginning or end of a dataset.

The Sadeh algorithm uses the y-axis epoch data. If any of the epoch counts are over 300, it reduces them to 300.

The algorithm requires the following information about the window that you're looking at:

  •  Arithmetic mean (average) of the activity counts for the window (AVG)
  • Number of epochs that have counts >= 50 and < 100 (NATS)
  • Standard deviation for the first 6 epochs of the window (SD)
  • Natural (base e) logarithm of a current epoch. Note: If the epoch count is 0, we make this value 0 to avoid infinity problems (LG)

Those calculations are put through the following algorithm:

(7.601 - (0.065 * AVG) - (1.08 * NATS) - (0.056 * SD) - (0.703 * LG)) 

If the result of that algorithm is GREATER than -4, then current epoch is considered asleep.

 

The Tudor-Locke Method


Tudor-Locke uses the sleep/wake results from either Cole-Kripke or Sadeh (whichever is applied for the study) and applies rules to them to detect sleep periods within the data. The parameters used within CentrePoint are defined below:

Bedtime Definition - 5 consecutive minutes of sleep. This determines the start of a sleep period.

Wake Time Definition - 10 consecutive minutes of awake time. This determines the end of a sleep period.

Minimum Sleep Period Length - 160 minutes. If the period would last for less than 160 minutes, it is not classified as a sleep period.

Maximum Sleep Period Length - 1440 minutes (24hrs). If the period would last for more than 1440 minutes, it is not classified as a sleep period. This helps mitigate false sleep periods in the instance that a device was removed.

Minimum Non-Zero Epochs - This option by default is disabled, but if enabled the default value is set to 15 minutes. This determines the minimum number of epochs (minutes) within the period that must have greater than zero activity to be considered a sleep period. These 15 minutes do not have to be consecutive. Similar to the Maximum Sleep Period Length parameter, this also helps mitigate false sleep periods by requiring at least 15 epochs within the sleep period that have activity counts greater than zero.

If you need any further assistance, visit our support portal at https://help.theactigraph.com/home. From there, you can view many helpful FAQ's and troubleshooting articles, log in to check any existing support requests, or create a new support ticket if you are unable to find a solution. You can also email us at support@theactigraph.com or give us a call at (877) 497-6996, Monday through Friday, 8 am to 5 pm CST.

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