Actigraph

An actigraph is an electronic device, that can be worn by an individual to perform actigraphy. An actigraph generally consists of a piezoelectric accelerometer, a filter (which filters out everything except the 2-3 Hz band, thereby ensuring external vibrations are ignored), a timer (to start/stop the actigraph at specific times, and to accumulate values for a specific time frame), a memory, to store the resulting values into, and an interface, usually USB or serial, to program the timer and download the data from memory. Most actigraphs also include a button so the user can indicate a specific event that occurs, for example that they go to bed.

Purpose
Actigraphs measurements can be used for different purposes:
 * Sleep: Sleep actigraphs are generally watch-shaped and worn around the wrist of the non dominant arm. They are useful for determining sleep patterns and circadian rhythms. They are generally worn for weeks at a time.
 * Activity: Activity actigraphs are worn and used similar to a pedometer: around the waist, near the hip. They are useful for determining the amount of activity and possibly the number of calories burned by the wearer. They are worn for a number of days.
 * Movement: Movement actigraphs are generally larger and worn on the shoulder of the dominant arm. They contain a 3D actigraph as opposed to a single dimension one, and have a high sample rate and a large memory. They are used for only a few hours, and can be used to determine problems with gait and other physical impairments.

Measurements
Actigraphs have a number of different ways of accumulating the values from the accelerometer in memory. ZCM (zero crossing mode) counts the number of times the accelerometer waveform crosses 0 for each time period. PIM (proportional integral mode) measures the area under the curve, and adds that size for each time period. TAT (time above threshold) uses a certain threshold, and measures the length of time that the wave is above a certain threshold. Literature shows that PIM provides most accurate measurements for both sleep and activity, though the difference with ZCM is marginal.

Features
Depending on the reason for your measurements, there are a number of features that you might want in your actigraph:
 * Fastest sample rate: 1 minute intervals provide adequate detail to measure sleep, but could be too slow for measuring other parameters.
 * Amount of memory: Together with sample rate the amount of memory determines how long you can measure.
 * Battery usage: Some actigraphs have a short battery life. This can also limit the time that you can measure. Replacing batteries often can be a hassle as well.
 * Weight: the heavier the actigraph, the more invasive its use. Especially in case of an actigraph being used for extensive periods this can pose a problem.
 * Water resistance: for proper measurements it is often good if the actigraph is worn in the shower, bathtub, or even while swimming/diving.
 * Additional features: watch functionality might increase acceptance of the actigraph, a query function might allow surveys at specific times, temperature measurement, skin resistance measurement.