Photoperiod is the number of hours of sunlight a point on Earth receives in a day. At the summer solstice, Madrid receives about 15 hours of light; at the winter solstice, only 9. At the equinoxes, day and night are roughly equal everywhere.
Biological influence
Photoperiod regulates plant flowering, bird migration, mammal hibernation, and reproductive cycles. In agriculture, it is fundamental: crops like soybean and rice are photoperiodic (flowering depends on day length). In humans, reduced winter photoperiod is associated with seasonal affective disorder (SAD).