“…Of all the reviewed articles, only 21 performed classification of all the sleep stages defined by either of these guidelines [ 24 , 39 , 42 , 45 , 50 , 58 , 67 , 72 , 76 , 80 , 88 , 89 , 90 , 93 , 101 , 102 , 105 , 108 , 109 , 117 , 121 ]. Amongst them, all but three [ 24 , 58 , 150 ] used EEG signals for classification, where the difference between sleep stages is known to be most obvious. In [ 150 ], where only PPG signals are used with accelerometry data to detect movements, the overall accuracy is quite low.…”