1. A method for estimating a frequency spectrum of a bio-signal that is non-uniformly spaced with low complexity, the method comprising the steps of: (a) sampling the bio-signal with a predefined sized window function; (b) replacing a sampled data value, within the window, at an arbitrary point by several data values on a regular mesh in such a way that sums over the mesh are an accurate approximation to sums over the original arbitrary point; (c) transforming the signal consisting of the extrapolated data samples of step (b) within the window in the wavelet domain using a wavelet transform by decomposing the signal into multiple frequency resolutions representing different classes of data by applying a series of suitable high-pass and low-pass filters based on the used wavelet basis; (d) processing the output data vector of the applied wavelet transform using reformulated Fourier operations with modified twiddle factors, values of the modified twiddle factors are obtained from the frequency response of the high-pass and low-pass filters of the applied wavelet transform; (e) selecting a threshold value such that the data obtained from the high-pass filters of the applied wavelet transform are pruned after applying the selected threshold; (f) eliminating the operations associated with the pruned high-pass data of step (e), in part or as a whole; (g) evaluating sine and cosine functions at the times corresponding to the data samples within the taken window using the output data obtained after applying steps (c), (d) and (e); (h) computing a time shift for each frequency in a desired set of frequencies to orthogonalize the sine and cosine components; and (i) computing the power of each frequency in a desired set of frequencies based on the estimated amplitudes of the evaluated sine and cosine functions.