How many hours should be in a day? One species of drosophila naturally prefers a 24.5 hour day, while another strongly prefers a 19-hour day. An animation and graph share data from each of these species when exposed to the typical light cycle of a 24-hour day as well as to no light when they must determine their own cycles. The patterns clearly demonstrate the importance of light in combination with an internal clock to determine the ideal length of a day.