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.
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