The Actigraphy is a sleep assessment tool recognized by the American Academy of Sleep Medicine (AASM) and the Taiwan Society of Sleep Medicine (TSSM). Actigraphy is equipped with an acceleration sensor, it is able to detect subtle sleep changes, while smartwatches only provide a roughly estimate of your sleep conditions.
In addition, an Actigraphy has a light sensor that records the source of light that the subject is exposed to, as light also affects the circadian rhythm, and most smartwatches do not detect the light source.