the sleep quality of the subject, such as his/her chronotype, the sleep onset latency, the total sleep duration, the sleep consolidation (sleep efficiency), the time spent in bed, movements, and the sleep cycle. Research showed that both sleep and wake are not equally assessed by actigraphy devices. When compared, data collected through polysomnographs and actigraphs defines sensitivity; which is the proportion of sleep correctly detected by both methods. Actigraphy reveals itself to be more likely to detect sleep than wake phases. Actigraphy has been actively used in sleep-related studies since the early 1990s. It has not traditionally been used in routine diagnosis of sleep disorders, but technological advances in actigraph hardware and software, as well as studies verifying data validity, have made its use increasingly common. The main reason for this development is the fact that, while retaining mobility, actigraphy offers reliable results with an accuracy that is close to those of polysomnography (above 90% for estimating total sleep time but dropping to 55% for a 4 - way sleep stage estimation problem). The technique is increasingly employed in new drug clinical trials where sleep quality is seen as a good indicator of quality of life. The technique has also been used in studies with individuals in both health and disease, e.g., Alzheimer's and fibromyalgia, conditions. Activity Activity actigraphs are worn and used similarly to a pedometer: around the waist, near the hip. They are useful for determining the amount of wake-time activity, and possibly estimating the number of calories burned, by the wearer. They are worn for a number of days to collect enough data for valid analysis. 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. The actigraph unit The unit itself is an integrate electronic device which generally embeds: an accelerometer, a real time clock or timer to start/stop the actigraph recording at specific times, and to record accumulate values for a specific time frame, a non-volatile memory to store the resulting values, an optional low-pass filter which filters out everything except the 2–3 Hz band, thereby ensuring external vibrations are ignored, and an interface, usually USB, serial,