Sleep discrepancy is the difference between between self-reported sleep duration and actigraphy-measured sleep. Actigraphy is increasingly being used to determine sleep patterns in normal, healthy adult populations, infants and children, and individuals suspected of certain sleep disorders. Actigraph devices are worn on the wrist and record movements that can be used to estimate sleep parameters with specialized algorithms in computer software programs. So basically sleep discrepancy is often regarded as a type of measurement error.
Sleep discrepancy has been well documented as occurring in individuals with insomnia. Sleep problems are common among older individuals, and it is thought that this discrepancy may represent a meaningful sleep-related phenomenon, which could have clinical significance.