A window above a heat vent or radiator may provide perfect light exposure, but the temperature and humidity levels may be bad. Light duration greatly influences plants, as flowering and rest periods are triggered by particular daylengths. Actually it is the length of the dark period, or night, that controls the response. For instance, Poinsettia blooms when given long nights. Blooming is prevented by a few minutes of light, given during the dark period.
Some plants enter rest periods during the fall and winter in response to shortening days.
Duration can partially compensate for low light intensity. Plants growing in low light may be more tolerant, if the duration of the light is longer. This is often done with artificial lighting. However, plants generally do not do well in continuous light. Altering the daylength may disrupt the blooming cycle of flowering plants, or, it may be used to bring them into bloom at a particular time.