The actigraph unit
The unit itself is an electronic device which generally consists of:
- a piezoelectric accelerometer,
- a low-pass 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, and
- an interface, usually USB, serial, or low-power wireless, to program the timer and download the data from memory.
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.
Actigraph units vary widely in size and features and can be expanded to include additional measurements. However, there are a number of limiting factors:
- 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 measurements can be taken.
- Battery usage: Some actigraphs have a short battery life.
- Weight: the heavier the actigraph, the more disruptive its use.
- Water resistance: for proper measurements it is often desirable that the actigraph be worn in the shower, bathtub, or even while swimming/diving.
For some uses, the following are examples of additional features:
- Watch functionality: making the device more attractive to the user.
- User input: most actigraphs now include a button so the user can indicate a specific event that occurs, for example lights out at bedtime.
- Subjective user input: for example a query function to allow surveys at specific times.
- Sensors which monitor:
- ambient light
- sound levels
- parkinsonian tremor
- skin resistance
- a full EEG data stream.