1. A method of providing a graphical representation of sleep quality, comprising: obtaining ECG data for a patient in a sleep assessment module of a computer processing unit; obtaining a plurality of N-N intervals from the ECG data in the sleep assessment module by 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; calculating in the sleep assessment module 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 the graphical representation of sleep quality in the sleep assessment module using the plurality of spectral densities, wherein the graphical representation is a plot having a time axis and a frequency axis, wherein each spectral density is used to create an image in the graphical representation, and wherein each image is color coded in a manner wherein a particular color at each frequency value along the frequency axis is determined based on an associated power at that frequency from the spectral density used to create the image.