A method of providing a graphical representation of sleep quality, comprising: obtaining, in a processing unit, ECG data for a patient; obtaining, in the processing unit, a plurality of N-N intervals from the ECG data; calculating, in the processing unit, a plurality of spectral densities based on the plurality of N-N intervals, wherein each spectral density is associated with one of a plurality of successive time windows and is calculated based on certain ones of the N-N intervals associated with the one of the plurality of successive time windows; and generating, in the processing unit, the graphical representation of sleep quality using the plurality of spectral densities; characterized in that obtaining the plurality of N-N intervals from the ECG data comprises processing the ECG data to identify a plurality of normal beats, and extracting the plurality of N-N intervals based on the identified normal beats, wherein the graphical representation is a plot having a time axis and a frequency axis, wherein each of the plurality of time windows has an associated mid value representing a middle of the time window such that each spectral density is associated with one of the mid values, and wherein each spectral density is used to create an image in the graphical representation that is positioned in the graphical representation along the time axis at a time corresponding to the mid value associated with the spectral density, wherein each image in the graphical representation has a length along the frequency axis equal to a frequency range of spectral densities, wherein the successive time windows are sliding time windows defined by x time window sliding y, meaning that each time window will be x units in length, and each successive time window will be spaced y units from a beginning of the time window immediately preceding it, and wherein each image in the graphical representation has a width along the time axis equal to y units, and wherein the frequency axis includes a plurality of particular frequencies along a length thereof, wherein in each image, a particular color located at each one of the particular frequencies is determined based on an associated power at the one of the particular frequencies from the spectral density used to create the image. obtaining, in a processing unit, ECG data for a patient; obtaining, in the processing unit, a plurality of N-N intervals from the ECG data; calculating, in the processing unit, a plurality of spectral densities based on the plurality of N-N intervals, wherein each spectral density is associated with one of a plurality of successive time windows and is calculated based on certain ones of the N-N intervals associated with the one of the plurality of successive time windows; and generating, in the processing unit, the graphical representation of sleep quality using the plurality of spectral densities; characterized in that obtaining the plurality of N-N intervals from the ECG data comprises processing the ECG data to identify a plurality of normal beats, and extracting the plurality of N-N intervals based on the identified normal beats, wherein the graphical representation is a plot having a time axis and a frequency axis, wherein each of the plurality of time windows has an associated mid value representing a middle of the time window such that each spectral density is associated with one of the mid values, and wherein each spectral density is used to create an image in the graphical representation that is positioned in the graphical representation along the time axis at a time corresponding to the mid value associated with the spectral density, wherein each image in the graphical representation has a length along the frequency axis equal to a frequency range of spectral densities, wherein the successive time windows are sliding time windows defined by x time window sliding y, meaning that each time window will be x units in length, and each successive time window will be spaced y units from a beginning of the time window immediately preceding it, and wherein each image in the graphical representation has a width along the time axis equal to y units, and wherein the frequency axis includes a plurality of particular frequencies along a length thereof, wherein in each image, a particular color located at each one of the particular frequencies is determined based on an associated power at the one of the particular frequencies from the spectral density used to create the image.