1. Method for estimating a hemoglobin-related blood constituent concentration of a user under low perfusion conditions: measuring a plurality of photoplethysmographic (PPG) signals by using a spectrophotometry-based monitoring device comprising a multichannel sensor formed from a plurality of channels, each PPG signal being measured by one of the channels; measuring a cardio-synchronous (CV) signal by using a cardiovascular sensor of the spectrophotometry-based monitoring device; and using a signal processor configured for: detecting an instantaneous heart rate and determining a heart rate variability from the CV signal; for each detected heart rate, determining projected PPG signals by projecting the measured PPG signals multiplied by a window function on a principal frequency component, and determining a magnitude of the projected PPG signals as the sum of the projected PPG signals; selecting reliable projected PPG signals, wherein reliable projected PPG signals are projected PPG signals which have a predetermined magnitude and a predetermined phase and/or coherence between the PPG signals and the principal frequency component; estimating a value of said blood constituent concentration from the magnitude of said reliable projected PPG signals selected over a predetermined time period; said window function having a window length which is adjusted at each detected heart rate according to the determined heart rate variability; and the frequency of said principal frequency component being estimated by averaging the heart rate over the window length.