*Geek Box: Actigraphy & Polysomnograpny
Actigraphy devices are worn like wrist watches, and provide an indirect assessment of sleep that is calculated through scoring systems which estimate sleep and wake time, and therefore additional parameters, largely from movement. Actigraphy devices estimate sleep as immobility, which may bias the actual results, however, the use of actigraphy has primarily been validated to assess sleep in free-living, naturalistic environments [although technically, they don’t measure ‘sleep’, but an assumption of sleep based on activity, and immobility].
Conversely, polysomnography [PSG] is the current gold standard for objective measures of sleep, however the complex nature of the technology confines the use of PSG to laboratory studies. A number of studies directly comparing PSG to actigraphy have found good correlation between sleep efficiency [% of total sleep time spent asleep], sleep latency [time to fall asleep], actual wake and sleep time.
However, an issue which may arise in relation to the use of actigraphy is an overestimation of sleep time, and underestimation of wake time. This measurement error may be derived from the fact that actigraphy estimates the onset of sleep as immobility, and because the device is worn on the wrist, depending on an individuals sleep habits it may look like there is less, or more, movement during the night.
Actigraphy is an important method, limitations aside, as it allows for field studies to be conducted with useful data on activity levels during the day, night, and can also quantify light exposure. This can be helpful as a condition of entry to a laboratory study, to ensure that participants complied with any recommended sleep-wake timing and light-dark exposures. In a laboratory study, however, if objective measures of sleep quality are desired, then PSG is the current gold standard.