Abstract: A method for evaluating a sleep quality is provided. The method includes the steps of acquiring a respiratory signal and a heart rate signal, capturing more than two consecutive windows of equal time length from the respiratory signal and the heart rate signal respectively, and performing a cross-spectrum analysis on a time-aligned respiratory window and a heart rate window, and obtaining a cross-spectrum amplitude matrix of the consecutive windows, constructing a stability coefficient matrix according to a cosine similarity between the cross-spectral amplitude matrix of the consecutive windows and estimating the sleep quality according to the stability coefficient matrix.