For the purpose of obtaining information about a person's sleep and wake states, an arrangement (100) comprising a video camera (10) and a processing unit (20) is used. The video camera (10) serves for capturing a sequence of video frames during a time period, and the processing unit (20) is configured to process video frames provided by the video camera (10) and to provide output representative of the person's sleep and wake states during the time period. In particular, the processing unit (20) is configured to execute an algorithm according to which (i) a motion value-time relation, (ii) sets of features relating to respective epochs in the motion value-time relation and (iii) classifiers of the respective epochs are determined, wherein the algorithm is further configured to apply an adaptive prior probability determined for the particular person in dependence of the motion values of the respective epochs to the classifiers.